Article

Sunset and the orientation behaviour of migrating birds

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Abstract

1. Migratory birds integrate information from a wide array of environmental sources. As our knowledge of migratory orientation depends heavily upon the results of cage-experiments with nocturnal migrants, it is essential that the results of these cage studies be interpreted in the light of field observations of migratory behaviour and experiments with free-flying migrants. When this is done, the impression emerges that night-migrating birds integrate directional information prior to departure, probably during the transition between daylight and darkness. At this time, information gained from the sun, in conjunction with other references, becomes especially valuable.

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... Furthermore, some data suggests a large intra-specific variation in this form of migratory behaviour (Cochran et al. 1967, Bolshakov 1992, Bolshakov & Rezvyi 1998, Åkesson et al. 1996, Moore & Aborn 1996). The timing of departure in small passerine nocturnal migrants may be determined not only by availability of celestial cues necessary for orientation during sunset and at twilight, but by variation in the atmospheric structure and energy condition of migrants (Moore 1987, Kerlinger & Moore 1989, Moore & Aborn 1996). It may reflect the interplay of endogenous time programmes with LD period changes over the season along migratory route, and also the position of birds in respect to the goal of their migration and migratory speed under different winds (Bolshakov & Rezvyi 1998). ...
... Thus they had access to various celestial information. The ability of birds to retain orientation notions (and possibly high motivation to fly), within first 24hrs after being denied access to celestial cues was shown both in the field and in experiment (Hebrard 1972, Helbig 1991, Moore 1987). Our results, and also our direct observations between sunset and darkness (Bolshakov & Rezvyi 1982, 1998 ), confirm the results of experimental studies that visual orientation cues, associated with sunset and star patterns, are important for nocturnal passerine migrants (Emlen 1975, 1980, Able 1980, Moore 1987, Wiltschko & Wiltschko 1991, Katz 1985, Helbig 1991). ...
... The ability of birds to retain orientation notions (and possibly high motivation to fly), within first 24hrs after being denied access to celestial cues was shown both in the field and in experiment (Hebrard 1972, Helbig 1991, Moore 1987). Our results, and also our direct observations between sunset and darkness (Bolshakov & Rezvyi 1982, 1998 ), confirm the results of experimental studies that visual orientation cues, associated with sunset and star patterns, are important for nocturnal passerine migrants (Emlen 1975, 1980, Able 1980, Moore 1987, Wiltschko & Wiltschko 1991, Katz 1985, Helbig 1991). In our opinion, the prolonged stay (2.5-3hrs) of inactive birds on the ground during twilight, may allow them not only to make a more " reliable " decision whether to migrate or not, but also to specify migratory direction, possibly estimate the distance to the goal of migration, and to solve other orientation tasks. ...
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Bolshakov, C.V. & V.N. Bulyuk (1999): Time of nocturnal flight initiation (take-off activity) in the European Robin Erithacus rubecula during spring migration: direct observations between sunset and sunrise. Avian Ecol. Behav. 2: 51-74. Temporal distribution of migratory nocturnal departures of the European Robin Erithacus rubecula, from a stopover site on the Courish Spit of the Baltic Sea, was studied systematically by direct observations (visual and searchlight methods), from sunset to sunrise over 7 years across the whole period of spring migration. Time of first take-off in spring is not fixed, day-to-day variation reaching 5.5 hours. First birds do not depart until 29-30 min after sunset. In 50% of cases spring nocturnal migration does not commence before the third hour after the sunset. The onset of nocturnal migration varies across the whole season, variation being 4.0 to 5.2 hours. The mean time of start of migration significantly shifts over the season by 45-75 min towards sun-set. During "long" nights (10-12 hours), in the first half of the migratory season, in 33% of cases the first birds do not depart until the fourth-sixth hour after sunset. During "short" nights (8-10 hours), in the second half of the season, in 95% of nights migration starts during first-third hour after sunset. Timing of nocturnal departures of Robins in spring is not synchronised and it is usually not possible to define the main departure period. Overall length of departure period reaches 9.5 hours, the latest take-off being re-corded in the tenth hour after sunset. Only 26.1% of Robins initiate flight before the end of nautical twilight (ENT). Approx. 42.5% of birds depart during the "deep" night after the end of astronomical twilight (EAT). The mean departure time in spring is 191 (SD=121) min after sunset. Only in some nights at the end of the migratory period do nearly all Robins depart during a restricted period after sunset and during twilight. All temporal characteristics of take-off activity are subject to significant changes over the season when night duration falls from 12 to 8 hours. Overall departure period changes from 9.5 to 6 hours, the mean time of nocturnal departures shifts towards sunset by more than one hour, the proportion of birds starting flight after EAT decreases, and the proportion departing before ENT increases. Inter-annual variation of temporal distri-butions of nocturnal departures, are probably related to the variation in the timing of spring migration. Within the same part of season however, inter-annual variation of the mean departure time may be over 1 hour. Variation of the mean and median departure time between successive nights of one year may also reach 1-1.5 hours. In spring Robins prefer to start migration under clear skies or slight cloud cover. As few as 9% of all birds de-parted without celestial cues visible under total overcast. Under cloudy skies, compared to clear weather, over-all departure period is longer together with the variation of the timing of departures, no seasonal trend in the mean time of start of migration and in the mean departure time is recorded and nocturnal migration in the first half of the season starts significantly earlier. The mean departure time under clear skies and high cloud scores is however, not significantly different either over the whole spring, or over fractions of the migratory period. Large variations in the timing of nocturnal departures is supposed to be caused by birds taking off, not in re-sponse to any environmental stimulus, but in respect to their individual time programmes. These programmes are a complicated manner related to seasonal LD changes and possess their own seasonal variation. A crucial factor controlling the time of nocturnal departure in spring may be the distance to the goal of migration. Con-siderable variation in the departure time basically referring to individuals approaching the goal (less then one night of flight).
... Numerous small songbirds fly during the night and make stopovers of varying duration to refuel (Dolnik 1975(Dolnik , 1995Alerstam & Lindström 1990). The commonly accepted model of temporal schedule of flights over land assumes for these birds: using the period of sunset as the main reference point; simultaneous take-off during Evening Nautical Twilight Period; flight during several hours; landing after midnight; no inter-or intraspecific variation (Moore 1987;Kerlinger & Moore 1989). Deviation from this basic pattern (except of departure period) may occur when migrants have to cross large expanses of water or deserts (Moreau 1972;Kerlinger & Moore 1989;Alerstam 1990;Biebach 1990;Martin 1990) or face unfavourable flight conditions (Biebach et al. 2000). ...
... It is assumed that the fixed time window of nocturnal departures soon after sunset is adaptive and allows the birds to use multiple cues to select a flight direction (Emlen 1980;Moore 1987;Able 1989Able , 1993Helbig 1991;Sandberg 1991). Such cues include calibration of magnetic and celestial compasses, based on sunset direction, skylight polarization pattern and stars (Able 1993;Able & Able 1995;Wiltschko et al. 1998;Cochran et al. 2004; Muheim et al. 2006). ...
... The data available do not allow us to claim that sunset is the main reference point controlling temporal schedule of nocturnal departures. However, we cannot rule out the possibility that before take-off, grounded migrants can use different celestial cues for selecting flight direction (Emlen 1980;Moore 1987;Able 1989Able , 1993Helbig 1991;Sandberg 1991;Able & Able 1995;Wiltschko et al. 1998;Cochran et al. 2004). The availability of celestial cues before departure is also important for making a decision to fly or not in a particular night (Å kesson et al. 2001). ...
Article
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To identify the mechanisms of control of temporal schedule of nocturnal migratory flights in small song- birds in nature, we studied the time of departure in a medium-distance nocturnal migrant, the European robin, on the Courish Spit (southeastern Baltic coast) using radiotelemetry. Exact measurements of depar- ture time in 100 birds (58 in autumn and 42 in spring) showed no fixed time window of nocturnal depar- tures during Evening Nautical Twilight. Take-offs occurred throughout the night and even during Morning Astronomical and Nautical Twilight Periods. Departure time showed no significant difference between clear skies and overcast conditions, and significantly differed between the seasons. The median departure time shifted significantly towards sunset during shorter nights in spring as compared to autumn. In both seasons (1) European robins initiating flight after short (1e2 days) stopovers showed no significant differ- ences in departure time between individuals with small and large fuel stores at arrival; and (2) departure time was significantly related to stopover duration: the longer the birds stayed, the earlier they departed in relation to sunset. Only in spring and after longest stopovers, all departures occurred within a short time window soon after sunset.
... This compass mechanism seems to be highly flexible, and differences in rates of sun azimuth changes during different hours of the day and at different latitudes and seasons may be taken into account [10,11,[38][39][40][41]. Birds may use such a time-compensated sun compass also for orientation at the times of sunset (in the case of nocturnal migrants) or sunrise (in the case of diurnal migrants) [12,42] (see section on time-compensated sunset compass below). Alternatively, birds could orient at a fixed angle relative to sunset or sunrise without time compensation (menotaxis) [43]; see section on fixed (menotactic) sunset compass below). Day migrants could theoretically take sun-compass readings once a day at noon or once an hour during the light hours of the day (see sections on time-compensated noon and time-compensated hourly sun compass below). ...
... Birds are assumed to use their time-compensated sun compass to establish their orientation around sunset [12,42,46]. We focus on sunset here, since a large proportion of passerines migrate at night and are believed to establish their departure direction around sunset [12,22,43,47]. For diurnal migrants, a time-compensated sunrise compass would work equally well. ...
... Birds following fixed (menotactic) sunset compass routes are assumed to orient at a fixed angle in relation to the local sunset azimuth throughout the migratory journey [43,47,49,50]. Hence, the flight course will change according to the change in sunset azimuth along the bird's migration route at the seasonal time of the bird's passage. ...
Article
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Birds use different compass mechanisms based on celestial (stars, sun, skylight polarization pattern) and geomagnetic cues for orientation. Yet, much remains to be understood how birds actually use these compass mechanisms on their long-distance migratory journeys. Here, we assess in more detail the consequences of using different sun and magnetic compass mechanisms for the resulting bird migration routes during both autumn and spring migration. First, we calculated predicted flight routes to determine which of the compasses mechanisms lead to realistic and feasible migration routes starting at different latitudes during autumn and spring migration. We then compared the adaptive values of the different compass mechanisms by calculating distance ratios in relation to the shortest possible trajectory for three populations of nocturnal passerine migrants: northern wheatear Oenanthe oenanthe, pied flycatcher Ficedula hypoleuca, and willow warbler Phylloscopus trochilus. Finally, we compared the predicted trajectories for different compass strategies with observed routes based on recent light-level geolocation tracking results for five individuals of northern wheatears migrating between Alaska and tropical Africa. We conclude that the feasibility of different compass routes varies greatly with latitude, migratory direction, migration season, and geographic location. Routes following a single compass course throughout the migratory journey are feasible for many bird populations, but the underlying compass mechanisms likely differ between populations. In many cases, however, the birds likely have to reorient once to a few times along the migration route and/or use map information to successfully reach their migratory destination. Electronic supplementary material The online version of this article (10.1186/s40462-018-0126-4) contains supplementary material, which is available to authorized users.
... It is widely believed that migratory birds use multifactorial navigation mechanisms which is defined as using several different cues for navigation Mouritsen, 2015;Wiltschko and Wiltschko, 1996). These can be based on visual (Cochran et al., 2004;Emlen, 1975;Griffin, 1952;Holland, 2003;Kramer, 1953;Moore, 1987;Muheim, 2011;Sauer, 1957), olfactory (Bonadonna and Bretagnolle, 2002;Bonadonna and Gagliardo, 2021;Safi et al., 2016;Wikelski et al., 2015), pressure (O'Neill, 2013), auditory (Hagstrum, 2013;Hagstrum et al., 2000) or geomagnetic cues (Kramer, 1953;Phillips, 1996) but less is known how these mechanisms interact or function together. One of the most debated navigational mechanisms is based on the Earth's magnetic field. ...
... landmark-based-navigation). Research has mostly focused on how animals may use these mechanisms independently (Åkesson and Hedenström, 2007;Gagliardo, 2013;Moore, 1987;Wikelski et al., 2015), either focusing on a single navigational strategy (Hore and Mouritsen, 2016;Wiltschko and Wiltschko, 2015) or on one geomagnetic value (Muheim et al., 2002;Ritz et al., 2000;Schiffner and Wiltschko, 2011;Wiltschko and Wiltschko, 2013). Combinations of different navigational mechanisms have been addressed in several modelling studies (Åkesson and Bianco, 2015;Zein et al., 2021). ...
Article
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Birds rely on precise navigational mechanisms, especially for long-distance migrations. One debated mechanism is their use of the geomagnetic field. It is unclear if and how different species of birds are using intensity or inclination (or both) for navigation. Previous geomagnetic modelling research is based on static geomagnetic data despite a temporally and spatially varying geomagnetic field. Animals supposedly have a high sensitivity to those changes of the geomagnetic field. In order to understand how birds respond in real-time to its temporal variation, we need to use accurate geomagnetic information linked to the position of the bird through co-location in space and time. We developed a data-driven approach to simulate geomagnetic migratory strategies, using, for the first time, accurate contemporaneous geomagnetic data obtained from Swarm satellites of the European Space Agency. We created biased correlated random walk models which were based on both GPS data from greater white-fronted geese (Anser albifrons) during fall migration between north-west Russia and central Europe and contemporaneous satellite geomagnetic data. Different strategies of geomagnetic navigation associated with different geomagnetic values were translated into probability surfaces, built from geomagnetic data, and included into the random walk models. To evaluate which strategy was most likely, we compared the measured GPS trajectories to the simulated trajectories using different trajectory similarity measurements. We propose this as an approach to track many bird species for future comparative studies. We found that navigational strategies in these geese using magnetic intensity were closer to the observed data than those using inclination. This was the case in 80% of the best models and is an indication that it should be more beneficial for these geese to use intensity over inclination. Additionally, our results supported results from a previous study, that navigation based on taxis and compass mechanisms were more similar to the observed data than other mechanisms. We therefore suggest that these geese may use a combination of these strategies for navigation at a broad-scale. Overall, it seems likely that for successful navigation to the target location more than one mechanism is necessary; indicating a multifactorial navigation mechanism of these migratory geese in the study area. The satellite geomagnetic data are available at a higher temporal resolution and the use significantly improved the fit of the modelled simulations in comparison to the modelled geomagnetic data. Therefore, using annotated geomagnetic data could greatly improve the modelling of animal geomagnetic navigation in future research.
... Billions of songbirds migrate from their wintering quarters to breeding quarters each year with their extraordinary navigation capabilities, covering several thousand kilometers following their endogenous circannual clock. Migratory birds take advantage of various compass systems so as to perform orientation throughout the migration, which includes information obtained from the earth's magnetic field [1][2][3][4][5][6][7], star patterns [3,[8][9][10][11][12][13][14], the sun's position at dusk/polarized skylight patterns [15][16][17][18][19][20][21], and retention of geographical cues en route [22]. The strength of the geomagnetic field was suggested as a cue for avian navigation in the 19th century [23]. ...
... Following long-distance songbirds, namely, wood thrushes (Hylocichla mustelina) and purple martins (Progne subis), through applying geolocators, researchers demonstrated that the general migration rate was two to six times faster in springtime compared with autumn [41]. Most passerine night migrants were supposed to calibrate their directional preferences around sunset [4,16,21,42]. The early takeoffs post-sunset during vernal migration than autumn [43] could likewise turn into an impact of birds to show up before the expected time towards the breeding quarters by increasing their migration speed [44]. ...
Article
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Red-headed buntings (Emberiza bruniceps) perform long-distance migrations within their southerly overwintering grounds and breeding areas in the northern hemisphere. Long-distance migration demands essential orientation mechanisms. The earth’s magnetic field, celestial cues, and memorization of geographical cues en route provide birds with compass knowledge during migration. Birds were tested during spring migration for orientation under natural clear skies, simulated overcast skies at natural day length and temperature, simulated overcast at 22 °C and 38 °C temperatures, and in the deflected (−120°) magnetic field. Under clear skies, the red-headed buntings were oriented NNW (north–northwest); simulated overcast testing resulted in a northerly mean direction at local temperatures as well as at 22 °C and 38 °C. The buntings reacted strongly in favor of the rotated magnetic field under the simulated overcast sky, demonstrating the use of a magnetic compass for migrating in a specific direction.
... Field crickets, Gryllus campestris, possess such and have been able to detect degrees of polarization as low as 5% (Herzman and Labhardt 1989), but it is unknown if for example birds have the same capability. Orientation cage experiments performed under natural overcast skies often result in scattered orientation, suggesting a reduced motivation to migrate when overcast, possibly due to an inability to navigate under these conditions (Moore 1987;cf. Åkesson et al. 1996. ...
... Evidences demonstrating the use of polarized light cues in migratory birds mainly come from experiments carried out around sunrise and sunset, which usually coincide with departure times of many migrants (Moore 1982(Moore , 1987Able 1993, 1995;Muheim et al. 2006cMuheim et al. , 2009reviewed by Muheim 2011). Migratory birds tested in orientation funnels with polarization filters on top of the cages change their orientation as a response to the shifted polarization axis, clearly indicating that they use the information from the polarized light transmitted though the filters to determine their preferred migratory direction (Able 1982, Helbig and. ...
... Precipitation, wind speed and direction, barometric pressure, fog, cloud cover, and temperature have all been shown to influence departure timing (reviewed in: Richardson 1978Richardson , 1990Liechti 2006;Shamoun-Baranes et al. 2017). Additionally, nocturnal migrants may regulate departure timing in response to light cues provided during sunset (Moore 1987;Sjöberg et al. 2017) as light conditions immediately after sunset may be particularly important for navigation via the earth's magnetic fields (Cochran et al. 2004). ...
... While it has long been known that white-crowned sparrows are nocturnal migrants in the spring, and nocturnal migratory restlessness has been observed in captive animals in both spring and autumn (Agatsuma and Ramenofsky 2006;Ramenofsky et al. 2008), this is the first field study to document autumn nocturnal departure in this species. Nocturnal departure may support navigation in flight, as there is some evidence that twilight could play a role in migratory orientation (Moore 1987;Cochran et al. 2004). An endogenous circadian clock entrained by sunset cues may also be used regulate timing of migratory departure (Coppack et al. 2008;Coppack and Bairlein 2011). ...
Article
Numerous studies of spring migration have discovered that environmental conditions experienced on the wintering grounds and/or during vernal migration itself can have “carry-over effects” altering timing or success of the subsequent breeding season. Few studies have evaluated whether breeding and pre-basic molt have carry-over effects on autumn migration. The aim of this study was to test the expectations that (1) at broad temporal scales carry-over effects from breeding and molt constrain migratory departure but that (2) at finer temporal scales local weather will further refine this decision. We monitored nests of Gambel’s white-crowned sparrows breeding in the low Arctic in Alaska over three years and used radio-telemetry to track autumn migratory departure. We found that reproductive timing and weather parameters, but not molt timing, contributed to variation in autumn departure from the breeding site. Birds that terminated parental care late in the summer departed from the breeding grounds late relative to other birds. Birds were more likely to leave on nights without precipitation, when barometric pressure was increasing, and when ground level south winds were prominent. We also observed that, on average, birds departed 2.5 h after sunset and shifted the hour of departure as sunset advanced over the migration season. Our findings, in conjunction with observations of migration from earlier studies, raise the possibility that global climate change may be delaying autumn migratory departure in Gambel’s white-crowned sparrows breeding in Alaska.
... Thus, the concept of mass departure of nocturnal passerine migrants within a limited period of dusk (Kerlinger & Moore 1989, Alerstam 1990, Berthold 1993 is not supported. Timing of take-off in small passerines is supposed to be related to various factors; availability of celestial cues necessary for orientation at sunset and at twilight, variation in the atmospheric structure, the energy condition of the migrants (Moore 1987, Kerlinger & Moore 1989, Moore & Abom 1996. It may reflect the interplay of endogenous time programmes with LD period changes over the season along the migratory route, and also the position of birds in respect to their migration destination and their migratory speed under different winds (Bolshakov & Rezvyi 1998, Bolshakov & Bulyuk 1999). ...
... Apart from endogenous factors, the decision to initiate migration can be influenced in passerine nocturnal migrants by the access to astronomical orientation cues at sunset and at nightfall (Moore 1987). In our experiments with freshly trapped Robins in spring more birds were night-active under overcast conditions in the evening, compared with limited or full access to celestial orientation cues. ...
Article
Bulyuk, V.N. & Mukhin, A. (1999): Commencement of nocturnal restlessness in the European Robin Erithacus rubecula during migration. Avian Ecol. Behav. 3: 79-90. We studied the time of onset of nocturnal migratory restlessness in the European Robin during spring and autumn migration. In 1996-1998 birds were specially trapped an hour or two before sunset in spring (10 April -20 May; n = 219) and in autumn (7 September-4 November; n = 287) and put in separate cages under the open sky with a view of sunset and the stars. Their behaviour was watched through infrared vision binoculars or recorded in automatic registration cages. Birds were released before sunset. In autumn, nocturnal restlessness was recorded in 18.1% of freshly captured Robins (15.7% and 22.6% in September and October, respectively). The proportion of Robins that displayed nocturnal activity was significantly positively related to their fat stores. Time of onset of nocturnal activity varied in different individuals between the second and the ninth hour after sunset (median 180 min). In September the median time was 155 min, in October 195 min. In November in four Robins out of seven nocturnal activity started five hours after sunset. In birds with large fat stores there was an observable trend to start nocturnal activity earlier in respect to sunset, than in leaner birds. In spring, nocturnal activity was recorded in 17.3% of freshly trapped Robins (16.7% and 18.4% in April and May, respectively). The time of onset of nocturnal restlessness varied between the second and the sixth hour after sunset (median 150 min). In late April the median was 147 min, in early May 153 min. Birds trapped during waves of passage (mass transit migration) tended to start nocturnal restlessness earlier in respect to sunset than conspecifics captured during migratory pauses. In birds held in captivity for some time, the proportion of active birds was much higher than in freshly trapped ones (65.8% in autumn, 60% in spring), the median being shifted towards the sunset: for 28 min in autumn and for 47 min in spring. The difference between freshly trapped and caged individuals are explained by higher fat scores of the latter, and by their delay on the migratory route. Variation in the timing of the onset of nocturnal activity in caged Robins is discussed, together with the relationship between behaviour in cages and migratory take-off activity in the wild.
... In some species, activity under starlight could result from the synchronization of directed behaviour with the brighter, more robust orientation cues at dusk. For example, many night-migrating birds take-off with the setting sun [4], to which they calibrate their magnetic compass [5], and can then use their star compass to maintain this established heading. A similar strategy may be adopted by nocturnal central-place foragers that leave their nest at sunset, such as sweat bees, ground spiders and ants [3,6,7]. ...
... On a succession of clear nights, however, celestial motion could drastically affect the capacity of a brightness-orienting animal to maintain a straight heading over greater distances. To adjust for celestial rotation, the animal might employ a time-compensated compass, or recalibrate its star compass relative to other references, such as the direction of sunset [4], wind [37], landmarks or magnetic cues [5]. ...
Article
Throughout history, the stars have provided humans with ever more information about our world, enabling increasingly accurate systems of navigation in addition to fuelling some of the greatest scientific controversies. What information animals have evolved to extract from a starry sky and how they do so, is a topic of study that combines the practical and theoretical challenges faced by both astronomers and field biologists. While a number of animal species have been demonstrated to use the stars as a source of directional information, the strategies that these animals use to convert this complex and variable pattern of dim-light points into a reliable 'stellar orientation' cue have been more difficult to ascertain. In this review, we assess the stars as a visual stimulus that conveys directional information, and compare the bodies of evidence available for the different stellar orientation strategies proposed to date. In this context, we also introduce new technologies that may aid in the study of stellar orientation, and suggest how field experiments may be used to characterize the mechanisms underlying stellar orientation.
... Investigations with several species of North American emberizine night migrants have shown that visual cues around the time of sunset are sufficient, if not indeed necessary, for meaningful migratory orientation (reviews, Able and Cherry, 1985;Moore, 1987). Experiments in which both magnetic directions and sunset position were shifted (the latter with mirrors) have been performed outdoors with Savannah sparrows (Passerculus sandwichensis) (Moore, 1985a). ...
... Migratory Savannah sparrows make their directional decisions around the time of sunset (review, Moore, 1987). Hand-raised sparrows given controlled exposure to the daytime sky under manipulated relationships of skylight polarization patterns and sun azimuth learned to perform orientation at dusk based on the e-vector of polarized light. ...
Article
Although only a small number of disparate taxa have been studied in detail, there are some general themes that transcend the orientation and navigation mechanisms of animals. I will identify some of these commonalities and illustrate them with data from selected species. Nearly all species rely on multiple environmental cues in orientation and these mechanisms seem often to be related hierarchically. Magnetic effects on orientation are widespread. The various cue-based orientation mechanisms are not independent, but influence one another during early development and later in life. The development of orientation capabilities involves complex interactions of experience and endogenous rules and learning predispositions. At the same time, considerable plasticity characterizes both the development of orientation and the behavior of adults in response to environmental situations requiring oriented movement.
... Veel diersoorten, waaronder ook bijen, maken gebruik van meerdere systemen voor hun navigatie (Wiltschko & Wiltschko, 2005;Emlen, 1975;Phillips, 1986, Moore, 1987Lohmann & Lohmann, 1996). Welk systeem ze wanneer gebruiken, hangt onder meer af van de situatie, het tijdstip op de dag en het weer. ...
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Veel volken van de honingbij overleven tegenwoordig de winter niet. Allerlei mogelijke oorzaken passeren de revue, waaronder de straling van het mobiele telefonie netwerk. Daar is weinig onder- zoek aan gedaan, maar het is bekend dat bijen magnetische en elektromagnetische velden kunnen detecteren en er op kunnen reageren. Het kan dus niet op voorhand worden uitgesloten dat bijen erdoor beïnvloed worden. Doelstelling van het onderzoek is om een relatie te kunnen leggen tussen aan de ene kant de dosis EMV die bijen in hun larvale en pupale stadium ondergaan en aan de andere kant hun ont- wikkeling en enkele kenmerken in hun volwassen leven (levensduur, vliegprestaties en morfolo- gische en fysiologische kenmerken). Ook wordt de ontwikkeling van het bijenvolk door het jaar heen (die een resultante is van de larvale, de pupale en de volwassen ontwikkeling) gemonitord.
... Compass orientation is particularly valuable for long-distance movements (such as migration, or navigation from unfamiliar places), because it allows the maintenance of goal-ward progress without the need to update position constantly. Naïve migrants may use innately instructed compasses to reach very distant overwintering sites [14][15][16][17], whilst for true navigation over unfamiliar terrain, Kramer's [18] two-stage map-and-compass concept has remained the dominant explanatory model [19,20]. Over short distances, and particularly over familiar terrain, the functional utility of compass orientation is less clear since landmarks can provide an independent source of directional guidance. ...
Article
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The sun has long been thought to guide bird navigation as the second step in a two-stage process, in which determining position using a map is followed by course setting using a compass, both over unfamiliar and familiar terrain. The animal's endogenous clock time-compensates the solar compass for the sun's apparent movement throughout the day, and this allows predictable deflections in orientation to test for the compass' influence using clock-shift manipulations. To examine the influence of the solar compass during a highly familiar navigational task, 24 clock-shifted homing pigeons were precision-tracked from a release site close to and in sight of their final goal, the colony loft. The resulting trajectories displayed significant partial deflection from the loft direction as predicted by either fast or slow clock-shift treatments. The partial deflection was also found to be stable along the entire trajectory indicating regular updating of orientation via input from the solar compass throughout the final approach flight to the loft. Our results demonstrate that time-compensated solar cues are deeply embedded in the way birds orient during homing flight, are accessed throughout the journey and on a remarkably fine-grained scale, and may be combined effectively simultaneously with direct guidance from familiar landmarks, even when birds are flying towards a directly visible goal.
... Variations in the time of departures in passerine nocturnal migrants depending on species, latitude, season, is obviously a complex problem (Akesson et al. 1996). Until now it has been discussed from the viewpoint of using of visual orientation cues, such as sunset and star pattern that are important for passerine nocturnal migrants (Emien 1975, Able 1980, Moore 1987, Wiltschko & Wiltschko 1991, dial variation of atmospheric structure (Kerlinger & Moore 1989), and energy stores of birds that start the migratory flight (Moore & Aborn 1996). Another consideration may appear not less important. ...
... Sedge warblers depart from their stopover sites around sunset, as do most nocturnal migrants (Moore, 1987;Zehnder et al., 2001;Åkesson et al., 2002). We considered meteorological data (wind and rain) during an eight-hour period starting at dusk, which varied across the season from 22:00 to 06:00 and 21:00 to 05:00 hrs. ...
Article
.—Bird migration is usually performed in several consecutive flights, interrupted by stopovers when birds rest or replenish their fuel loads. As a result, migrants must decide when and where to land. Here, we studied the effects of meteorological conditions (wind and rain) and age (used here as a indicator of bird experience) on the probabilities of sedge warblers Acrocephalus schoenobaenus landing at a stopover site in northern Iberia. Data were collected over three consecutive years at a ringing station during the autumn migration period. We used reverse-time capture-mark-recapture models to estimate seniority, γ (i.e., the probability that an individual at time t was already present in the population at time t -1), as an indicator of landing decisions, since 1-γ represents the probability of recording new individuals (i.e. recent landings). We ran 14 models with the above mentioned variables, four of which were best supported by the data. In these, only rain showed a significant positive effect on γ, indicating that birds of any age class avoid flying during rainfall and prefer to interrupt their migration. These results are similar to those obtained from an analysis of day-to-day variation in first captures that was used to validate the usefulness of capture-mark-recapture models. They suggest that CMR models can serve to study bird landing decisions during migration in some specific cases. RESUMEN.—Las aves realizan su migración en etapas alternativas de vuelos, interrumpidas por pe-riodos de parada en áreas de descanso. Como consecuencia, las aves deben tomar decisiones sobre cuándo y dónde parar a lo largo de la migración. Hemos estudiado los efectos de las condiciones mete-orológicas (lluvia y viento) y edad (como indicador de la experiencia de las aves) en las probabilidades
... Migrating birds use different cues for orientation, including the magnetic field of the Earth (Wiltschko and Wiltschko, 1972), the position of the sun at sunset (Moore, 1987; Schmidt-Koenig, 1990), light polarization patterns around sunset and sunrise (Able, 1982; Phillips and Moore, 1992; Able and Able, 1993; Muheim et al., 2006b) and the position of the stars (Wagner and Sauer, 1957; Wiltschko and Wiltschko, 1978; for reviews, see Muheim et al., 2006a; Wiltschko and Wiltschko, 2009). Since cue availability changes depending on a bird's location on the Earth, time of day and season, it seems likely that the multiple compass systems are regularly calibrated by a common reference system (Cochran et al., 2004; Muheim et al., 2006a; Muheim et al., 2006b), and that one of the multiple compasses is dominant over the others. ...
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The magnetic field, the sun, the stars and the polarization pattern of visible light during twilight are important cues for orientation in nocturnally migrating songbirds. As these cues change with time and location on Earth, the polarization pattern was put forward as a likely key reference system calibrating the other compass systems. Whether this applies generally to migratory birds is, however, controversially discussed. We used an experimental approach in free-flying birds to study the role of polarization for their departure direction in autumn. Experimental birds experienced a 90° shift of the band of maximum polarization during sunset, whereas control-birds experienced the polarization pattern as under natural condition. Full view of the sunset cues near the horizon was provided during the cue conflict exposure. Here we show both the experimental and the control-birds being released after nautical twilight departed consistently towards south-southeast. Radio telemetry allowed tracking first 15 km of birds' way out, thus the intrinsic migration direction as chosen by the birds was measured. We found no recalibration of the magnetic compass after pre-exposure to a cue conflict between the natural magnetic field and the artificially shifted polarization pattern at sunset. The lacking difference in the departure direction of both groups may suggests that birds did not recalibrate any of the compass systems during the experiment. As free-flying migrants can use all available orientation cues after release, it remains unknown whether our birds might have used the magnetic and/or star compass to determine their departure direction.
... This behaviour is more like that of a diurnal migrant in which migration typically starts up to 1 h before sunrise with peak migratory activity some 2 h later (Alerstam & Ulfstrand 1975). True night migrants typically take off within the first hour after sunset, with migratory activity reaching a peak for the next four hours (Moore 1987). Silvereyes are probably neither typical night migrants nor typical diurnal migrants, but somewhere in between. ...
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This study examined diurnal and nocturnal activity patterns from 40 hours of continuous activity recordings of individually caged resident and migrant Silvereyes Zosterops lateralis. Elevated night activity (= Zugunruhe) was displayed by migrants in autumn and spring but not by residents. Zugunruhe commonly commenced in the dark portion of the morning and continued to about 2-3 h beyond the onset of light. Migrants were more night restless than residents in winter and in the migratory seasons of autumn and spring. They were also more diurnally active in the winter months than residents. The increase in diurnal activity in the latter period is related to winter migratory movements.
... Sedge warblers depart from their stopover sites around sunset, as do most nocturnal migrants (Moore, 1987; Zehnder et al., 2001; Åkesson et al., 2002). We considered meteorological data (wind and rain) during an eight-hour period starting at dusk, which varied across the season from 22:00 to 06:00 and 21:00 to 05:00 hrs. ...
... During the autumn migration period it accumulates large fuel loads at suitable sites in Europe that offer a superabundant food supply, in particular the reed aphid Hyalopterus pruni, and then reaches its wintering areas south of the Sahara practically without refuelling (Bibby and Green, 1981;Schaub and Jenni, 2001a). As a typical warbler, it is a nocturnal migrant that initiates migratory flights around sunset (Moore, 1987;Åkesson et al., 1996). ...
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Factors determining departure decisions of migrants from a stopover site can be extrinsic and/or intrinsic but the relative role of each of these factors on departure decisions is still poorly known. Date and wind should be the main factors determining departure decisions in a long-distance migrant, which is expected to minimise duration of migration. Date was considered as an intrinsic factor and wind as an extrinsic one. We analysed the capture-recapture data of a long-distance migrant European songbird, the sedge warbler Acrocephalus schoenobaenus, from a stopover site in northern Iberia during the autumn migration period to quantify the relative importance of several factors on emigration likelihood. Cormack-Jolly-Seber (CJS) models were used to estimate the emigration likelihood. From 107 models tested, only the model with an additive effect of date, tailwind assistance and population size substantially supported the data. As expected, sedge warblers were more likely to depart with high tailwind values and late in the season and, contrary to expectations, with decreasing sedge warbler abundance.
... Migratory birds use multiple compass systems for orientation during migration, which include a magnetic compass Wiltschko, 1972, 1995), star compass (Emlen, 1970(Emlen, , 1975 and a sun/polarized skylight compass (Able, 1982;Moore, 1987;Schmidt-Koenig, 1990;Munro and Wiltschko, 1995). Because of changing relationships between the compass cues and altering cue availability due to weather conditions, time of day, season, and latitude, birds have to calibrate the different compasses with respect to a common reference on a regular basis (Bingman, 1983;Wiltschko et al., 1998;Able and Able, 1999;Muheim and Åkesson, 2002;Bingman et al., 2003;Muheim et al., 2006a). ...
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Migratory birds use multiple compass systems for orientation, including a magnetic, star and sun/polarized light compass. To keep these compasses in register, birds have to regularly update them with respect to a common reference. However, cue-conflict studies have revealed contradictory results on the compass hierarchy, favoring either celestial or magnetic compass cues as the primary calibration reference. Both the geomagnetic field and polarized light cues present at sunrise and sunset have been shown to play a role in compass cue integration, and evidence suggests that polarized light cues at sunrise and sunset may provide the primary calibration reference for the other compass systems. We tested whether migratory garden warblers recalibrated their compasses when they were exposed to the natural celestial cues at sunset in a shifted magnetic field, which are conditions that have been shown to be necessary for the use of a compass reference based on polarized light cues. We released the birds on the same evening under a starry sky and followed them by radio tracking. We found no evidence of compass recalibration, even though the birds had a full view of polarized light cues near the horizon at sunset during the cue-conflict exposure. Based on a meta-analysis of the available literature, we propose an extended unifying theory on compass cue hierarchy used by migratory birds to calibrate the different compasses. According to this scheme, birds recalibrate their magnetic compass by sunrise/sunset polarized light cues, provided they have access to the vertically aligned band of maximum polarization near the horizon and a view of landmarks. Once the stars appear in the sky, the birds then recalibrate the star compass with respect of the recalibrated magnetic compass. If sunrise and sunset information can be viewed from the same location, the birds average the information to get a true geographic reference. If polarized light information is not available near the horizon at sunrise or sunset, the birds temporarily transfer the previously calibrated magnetic compass information to the available celestial compasses. We conclude that the type of cue-conflict manipulation and the availability of stars can explain the discrepancies between studies.
... He suggested that suspension of activity allows for complete digestion of food reducing excess weight at take-off. Others have suggested that migrants obtain orientation cues from the setting sun (solar compass or polarized light) as they sit quietly at dusk (Krantz & Gauthreaux, 1975; Moore, 1987, Sandberg et al., 1991 Cochran et al., 2004 ). GWCS use solar and celestial cues for orientation during migration (Mewaldt, 1964; Åkesson et al., 1996, 2001). ...
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Gambel's white-crown sparrow (Zonotorichia leucophrys gambelii) is a long-distance, over-land migrant. In captivity birds display many characteristics of the autumn and spring migratory life history stages that include hyperphagia, fattening and high intensity nocturnal activity termed migratory restlessness or Zugunruhe. We recorded the behaviour of captive birds while simultaneously collecting 24 h locomotor activity. These data were used to define the behaviour displayed by captive birds during autumn and spring in order to compare the two migratory stages and to draw inferences for free-living birds. The predominant behaviour during day and nighttime was rest. Feeding occurred only during daylight hours but at a greater frequency in autumn than spring. Birds generally used their feet as the primary source of locomotion during the day termed 'jump'. During the night, two distinct behaviours, 'beak-up flight' and 'beak-up' involving high intensity wing motions were observed and considered components of migratory restlessness. The frequency of the 'beak-up flight' was greatest during spring and associated with the enhanced tempo of vernal migration. In both stages, migratory restlessness was preceded by a quiescent phase, the occurrence of which differed and related to time available for foraging and length of the night. Given these findings, we hypothesize that diel behaviours displayed by autumn and spring migrants in captivity highlight distinctions between the two life history stages.
... As most migratory bird species travel exclusively at night (Bruderer, 1997;Dorka, 1966), the time after sunset at which a bird departs from stopover largely determines the duration of that night's flight bout. Traditionally, migrants were assumed to generally set off shortly after sunset (reviewed in Moore, 1987). Later radiotelemetry studies, however, revealed considerable variation in nocturnal departure time between individuals of the same species (Åkesson, Alerstam, & Hedenstr€ om, 1996;Bolshakov & Chernetsov, 2004;Moore & Aborn, 1996;Smolinsky, Diehl, Radzio, Delaney, & Moore, 2013). ...
Article
Migrating animals typically make stopovers to rest and replenish the fuel used during previous travel bouts. In birds, fat or fast refuelling individuals are generally more likely to depart from stopover than lean or slow refuelling birds. The departure decision, however, involves more than the day-to-day departure likelihood. Also, the time of night (or day) at which migrants depart is relevant, as this largely determines the duration of the flight bout and thereby affects the speed of migration. Because stopovers serve to replenish fuel stores, longitudinal data are most informative regarding the relationship between fuel stores and nocturnal departure time, but no such data exist. To fill parts of this gap, we caught and temporarily caged migrating northern wheatears, Oenanthe oenanthe, at a spring stopover site. We related the birds' evening fuel stores to their start of nocturnal migratory restlessness. In northern wheatears, this is a reliable proxy for their actual departure time under free-flying conditions. We observed a strong negative within-individual effect of fuel stores on the start of migratory restlessness. With an increase in fuel stores birds advanced their start of restlessness. This clearly shows that evening fuel stores are important for migrants' timing of departure at night. Hence, next to the regulation by innate rhythms, the timing of nocturnal departures during spring migration also depends on cues from fuel stores.
... The majority of nocturnal migrants are known to depart within the first few hours after the 68 sunset (Liechti et al. 1997, Bulyuk and Tsvey 2006, Schmaljohann et al. 2011). This allows for compass 69 calibration at twilight for orientation (Moore 1987, Muheim et al. 2006) and for longer nocturnal 70 flights when larger distances can be covered. Landing typically occurs around sunrise (Bruderer and 71 Liechti 1999) giving a maximum nocturnal flight duration of ca. 12 h during autumn migration in the 72 Northern hemisphere in September and ca. 10 h during spring migration in April (Bauchinger and 73 Klaassen 2005). ...
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Migratory birds complete their seasonal journeys between breeding and non‐breeding sites with a series of migratory flights that are separated by prolonged stopovers. While songbirds are the most common taxa among migratory birds, empirical data on flight and stopover behaviour along their entire migratory journeys are still rare. Here, we integrate activity and barometric pressure tracking with classical light‐level geolocation to describe migration behaviour of tawny pipits Anthus campestris breeding in Central Europe. Surprisingly, tracked pipits used, on average, as many as 10 stopover sites during their six week, >5000 km long autumn migration. This conforms to a typical hop‐type pattern of migration. In contrast to common knowledge which considers the tawny pipit as a typical diurnal migrant, our data revealed that more than 2/3 of all migratory movements were carried out at night. Nocturnal departure times were highly variable within individuals and spread across the entire night while landing most often took place within the first few hours after sunrise. Consequently, there was a negative relationship between departure timing relative to sunset and flight duration. Short flights of up to 2 h were most common and median flight duration was 4.5 h. There was a hyperbolic relationship between flight duration and maximum flight altitude and flight altitudes during night were two times higher compared to daytime. The overall ratio of flight vs stopover duration during migration was on average 1:6.5. This closely matches predictions from theoretical models. We show that multi‐sensor tracking has the potential to provide unprecedented details on migratory behaviour of individual birds along their entire migratory journeys, and it also improves the precision of geographical locations derived from light‐level geolocators. This article is protected by copyright. All rights reserved.
... Migrating birds are able to use different compass mechanisms based on geomagnetic (Wiltschko and Wiltschko 1996) or celestial cues, including the sun (Kramer 1953;Moore 1987), stars (Emlen 1975) and skylight polarisation pattern (Able and Able 1995;Cochran et al. 2004; ...
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Despite our extensive knowledge on various aspects of their lives, there has been limited investigation into the hierarchical relationships among different compass systems in shorebirds. The aim of this study was to investigate the relationship between magnetic and celestial compasses in two species of shorebirds, the curlew sandpiper (Calidris ferruginea; pre-breeding migration) and the dunlin (Calidris alpina; post-breeding migration) using cue-conflict experiments. Birds were captured in a Mediterranean stopover site, after which their magnetic orientation was determined under simulated overcast conditions at sunset using modified Emlen funnels fitted with infrared video cameras. Birds that demonstrated a well-defined directional preference were then exposed over two sunsets to conflicting directional information between the local geomagnetic field and the ±90° shifted band of maximum polarisation. These individuals were tested again for magnetic orientation at sunset in the same conditions as previous test, to determine whether their directional choices had changed after the cue-conflict. Our results showed that individuals from both species did not recalibrate their magnetic compass from visual cues after the cue-conflict, even though at least dunlins did not appear to completely disregard the information derived from celestial cues. This study is one of the few experimental studies on the migratory orientation of Charadriiformes and on the hierarchical relationships between the different compasses used by these birds during their extensive migratory movements. Significance statement Migrating birds are able to use different compass mechanisms based on geomagnetic or celestial cues, and it seems reasonable to hypothesise that birds calibrate their various compasses to maintain the correct direction especially when the directional information does not agree. The hierarchy among different compasses has been studied largely on night migrating passerines, but it is still poorly understood. We investigated the hierarchy among geomagnetic and celestial cues (band of maximum polarisation) in two species of Charadriiformes by means of cue-conflict experiments. Our result showed that the geomagnetic cues have a dominant role in the orientation mechanisms of the studied species, even though the information derived from celestial cues did not appear to be completely disregarded.
... The different twilight periods are related to differences in availability of celestial orientation cues: the position of the setting sun, under clear sky conditions, may be seen as a glow on the horizon even during nautical twilight; the pattern of polarized light from the sun is most prominent during civil twilight; the first and brightest stars become visible during the end of civil twilight, and after nautical twilight only information from stars remains (Rozenberg, 1966). Before songbirds depart from a stopover site they integrate directional information for orientation and possibly assess flight conditions (Able, 1982;Able & Able, 1993;Liechti, 2006; reviewed in Moore, 1987;Muheim, Moore, & Phillips, 2006;Muheim, Phillips, & Åkesson, 2006;. Several studies on departure timing in nocturnal songbird migrants have focused on departure times in relation to sun elevation and the availability of orientation cues associated with different twilight periods, with the hypothesis that birds depart as a response to the availability of a specific combination of orientation cues. ...
Article
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Most songbirds depart from stopover sites after sunset and migrate during the night. Several recent studies have reported larger variation in departure time than previously thought; yet, it is still unclear which factors govern departure timing. We investigated the departure timing of four species of nocturnally migrating songbirds using an automated radiotelemetry system at Falsterbo peninsula in southwest Sweden. We made a comprehensive analysis to test a range of factors that have been hypothesized to affect departure timing of nocturnal migrants, such as night duration, season, sun elevation and the birds' intrinsic and environmental conditions. We hypothesized that birds in good condition (large fuel reserves) and under advantageous flight conditions would depart sooner after sunset, in the expected migratory direction. Our analyses showed that the birds departed sooner after sunset during spring than autumn, and different species departed at different times in relation to sunset. In addition, birds departed earlier when nights were shorter, suggesting that night duration is an important factor that may drive much of the observed timing differences between seasons and species. Lean birds delayed their departures compared to fat individuals. When birds experienced favourable wind conditions (tail wind or weak winds) at sunset, they departed earlier. Thus, it appears that the decision to take off for a long-distance flight depends on both body condition and wind conditions. Timing of departure was not correlated with sun elevation, which would have been expected if availability of specific orientation cues (sun, skylight polarization pattern, stars) acted as triggers for departures. These results stress high flexibility and adaptive responses to a complex of ecological factors as the determinants for timing of nocturnal flights in songbirds.
... Moreover, nocturnally migrating flyers are: 1) less likely to move during the day (i.e. sleep instead of forage), 2) engage in quiescence and orientation behaviors before departing, and 3) depart stopover sites early at night between sunset and astronomical twilight to take advantage of improved directional information, atmospheric stability and lower predation risk (Moore 1987, Kerlinger and Moore 1989, Moore and Aborn 2000, Newton 2008, Németh 2009, Cohen et al. 2012, Covino and Cooney 2015, Dossman et al. 2016, Schofield et al. 2018a, b, Packmor et al. 2020. ...
Article
During migration, birds must locate stopover habitats that provide sufficient resources to rest and refuel while en route to the breeding or non‐breeding area. Long‐distance migrants invariably encounter inhospitable geographic features, the edges of which are often characterized by habitat limited in food and safety. In response, they often depart in directions inconsistent with reaching their destination, presumably searching for better habitat. We used automated radio telemetry to track 442 individuals of five species to investigate the behavior of migratory birds as they departed edge habitat along the northern Gulf of Mexico coast during autumn from 2008 to 2014. Most migrants (75%) retreated inland or detoured around rather than advanced across the Gulf, but this depended on bird species and fat‐based energy stores. Most individuals in lean condition or of smaller bodied species tended to retreat or detour, rather than advance, when departing from the coast. Twenty‐one percent of all birds that departed the coast in 2013–2014 were redetected over 45 km inland, providing a unique opportunity to compare stopover duration, departure times and travel speeds between migrants that retreat away from the coast and those that continue to advance toward their destination. Individuals that retreated the coast and were redetected inland spent ~1 day on the coast before retreating inland, where they spent 11 days before resuming migration. Further when those same individuals retreated from the coast, they departed around evening civil twilight, whereas those that advanced from inland habitats departed after evening civil twilight. Travel speeds were slower for individuals retreating inland compared to those advancing towards the coast from inland habitats. The differences between retreating and advancing individuals suggest how an individual's drive to feed or fly influences behavior. Our study illustrates how the sum of individual decisions can shape habitat use, landscape‐scale movements and migration strategies.
... However, unlike many passerines that migrate through the night and use the daylight hours for rest and foraging, many of the Whimbrels departing from the study area will be flying non-stop for several days to reach breeding areas (Watts et al. 2008). Moore (1987) has suggested that the orientation cues available to migrants around sunset are particularly beneficial for navigation. Despite the uncertainty of the benefits of evening departure, the pattern described in this paper is particularly strong. ...
Article
Many Arctic-breeding shorebirds utilize terminal spring staging areas that are strategically positioned within mid-temperate latitudes. Because these staging areas represent final refueling opportunities and have a direct link to the breeding grounds, the condition of birds and their pattern of departure may influence reproductive performance. A significant portion of Canada’s Hudson Bay and Mackenzie River Whimbrel populations utilize the tidal salt marshes and mudflats of the mid-Atlantic coast of North America as a terminal stopover site. We observed Whimbrels (n = 39,720 individuals) departing from the lower Delmarva Peninsula in Virginia (USA) during the second half of May (2009–2014) to quantify departure dates, time of day and flock size. The majority (90%) of Whimbrels departed the study area during the six-day period of 22–27 May with a clear peak during 24–25 May when mean departure rates exceeded 600 birds/hr. Departure date appears to be highly conserved with median date varying across only three days over six years. Departures peaked approximately 2.5 hrs before civil twilight with 82% of individuals leaving within the two-hr period between 1.5 and 3.5 hrs before twilight. The distribution of departing flock sizes approximated a negative exponential as smaller flocks were more common. Although 50% of all flocks recorded contained less than 35 individuals, 50% of all individuals occurred in flocks that contained 80 individuals or more. The highly synchronous and consistent departure pattern may help to facilitate synchronous arrival on the breeding grounds and reinforce mate fidelity. Group travel may provide benefits such as collective navigation and energetic savings related to flock aerodynamics.
... We realized that the influence of the combined weather factors is highly statistically significant (p-value of the F-test = 0.002) and undertook further analysis on each factor. Total Bright Sunshine (p-value = 0.031) is statistically significant in affecting bird activities, consistent with previous studies reporting that sunlight influences migration, foraging and other activities of birds [55][56][57] Prevailing Wind Direction (EW, p-value = 0.011) is also statistically significant, suggesting that wind has negative effect on nestlings survival 51 . ...
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Automatic bird detection in ornithological analyses is limited by the accuracy of existing models, due to the lack of training data and the difficulties in extracting the fine-grained features required to distinguish bird species. Here we apply the domain randomization strategy to enhance the accuracy of the deep learning models in bird detection. Trained with virtual birds of sufficient variations in different environments, the model tends to focus on the fine-grained features of birds and achieves higher accuracies. Based on the 100 terabytes of 2-month continuous monitoring data of egrets, our results cover the findings using conventional manual observations, e.g., vertical stratification of egrets according to body size, and also open up opportunities of long-term bird surveys requiring intensive monitoring that is impractical using conventional methods, e.g., the weather influences on egrets, and the relationship of the migration schedules between the great egrets and little egrets.
... The clock that times Zugunruhe is highly responsive to nutritional state and ticks more slowly than the daytime clock [38]. Although captivity findings generally corresponded well with field data, recent studies also revealed differences: whereas in captive migrants, Zugunruhe usually sets in shortly after sunset [39], free-living migrants adjusted the timing of departure depending on environmental factors and their body condition [40][41][42][43]. Furthermore, whereas in captivity, autumn Zugunruhe usually builds up by a gradual activity shift into the night, a recent study on free-living short-distance migrants revealed that the birds left instantaneously during their first restless night [44]. ...
Article
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Migratory birds regularly perform impressive long-distance flights, which are timed relative to the anticipated environmental resources at destination areas that can be several thousand kilometres away. Timely migration requires diverse strategies and adaptations that involve an intricate interplay between internal clock mechanisms and environmental conditions across the annual cycle. Here we review what challenges birds face during long migrations to keep track of time as they exploit geographically distant resources that may vary in availability and predictability, and summarize the clock mechanisms that enable them to succeed. We examine the following challenges: departing in time for spring and autumn migration, in anticipation of future environmental conditions; using clocks on the move, for example for orientation, navigation and stopover; strategies of adhering to, or adjusting, the time programme while fitting their activities into an annual cycle; and keeping pace with a world of rapidly changing environments. We then elaborate these themes by case studies representing long-distance migrating birds with different annual movement patterns and associated adaptations of their circannual programmes. We discuss the current knowledge on how endogenous migration programmes interact with external information across the annual cycle, how components of annual cycle programmes encode topography and range expansions, and how fitness may be affected when mismatches between timing and environmental conditions occur. Lastly, we outline open questions and propose future research directions. This article is part of the themed issue ‘Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals’.
... Factors that influence quiescence, much less its function(s), if any, are unknown. A period of inactivity (quiescence) may be used to obtain information pertinent to departure, including a birds' internal state (energetic condition, hormone levels), meteorological conditions, directional information, and geographical position in relation to destination (Emlen 1980;Moore 1987;Cochran et al. 2004;Ramenofsky et al. 2008;Chernetsov 2012;Goymann et al. 2017). For example, the calibration of a migrant's magnetic compass with polarized light, which is thought to take place at sunset and sunrise (e.g., Sjöberg and Muheim 2016), is probably most reliably made when the migrant is inactive (Muheim pers. ...
Article
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Quiescence is a period of inactivity that occurs before the onset of migratory activity in nocturnally migrating birds. This behavior has been observed in captive birds in migratory disposition, but its occurrence in free-ranging migratory birds has been documented only anecdotally, and causal factors and function(s), if any, are unknown. In this study, we documented and characterized quiescence in three migratory songbird species (red-eyed vireo [Vireo olivaceus], Swainson’s thrush [Catharus ustulatus], and wood thrush [Hylocichla mustelina]) by measuring movement and proportion of time spent inactive prior to departure from a stopover site during fall migration. Individuals of each species displayed a period of inactivity prior to departure which varied from less than 30 min to over 90 min with red-eyed vireos engaged in the longest, most pronounced quiescence. We also examined how quiescence was related to intrinsic and extrinsic factors known to influence the departure of migrating birds, and found some evidence for an effect of age and departure time but no effect of a migrant’s energetic condition, departure direction, atmospheric conditions around departure, or day of year on quiescence. Our novel application of an automated radiotelemetry system yielded a large amount of data to characterize quiescence in free-ranging migratory birds, and we provide guidance for future studies to tease apart the various causal factors and function(s) of this migratory behavior. Significance statement Quiescence is a poorly understood period of inactivity observed among captive and free-ranging migratory songbirds prior to the onset of nocturnal activity. Our novel use of automated radiotelemetry revealed quiescence among three intercontinental migratory songbirds. It also enabled us to ask how quiescence might be related to intrinsic and extrinsic factors known to influence the departure of migrating birds, and provided an opportunity to explore possible function(s), if any, of this intriguing behavior.
... For example, only 29 % of Swainson's thrushes captured by Woodrey and Moore (1997) at Fort Morgan peninsula in 1990–1992 were after hatch-year birds and few of these birds were lean. Timing and direction of departures: Nocturnally migrating birds typically initiate a flight around civil twilight, presumably because reliable directional information is available from multiple sources (Emlen 1980; Moore 1987; Ǻkesson et al. 1996) and because of the advantages (e.g., atmospheric stability; Kerlinger and Moore 1989) of nocturnal migration. Swainson's thrushes carrying sufficient fat stores to cross the Gulf of Mexico departed within a narrow window of time between sunset and astronomical twilight regardless of age, moving in a southerly direction out over the Gulf of Mexico. ...
Article
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Whether or not a migratory songbird embarks on a long-distance flight across an ecological barrier is likely a response to a number of endogenous and exogenous factors. During autumn 2008 and 2009, we used automated radio tracking to investigate how energetic condition, age, and weather influenced the departure timing and direction of Swainson’s thrushes (Catharus ustulatus) during migratory stopover along the northern coast of the Gulf of Mexico. Most birds left within 1 h after sunset on the evening following capture. Those birds that departed later on the first night or remained longer than 1 day were lean. Birds that carried fat loads sufficient to cross the Gulf of Mexico generally departed in a seasonally appropriate southerly direction, whereas lean birds nearly always flew inland in a northerly direction. We did not detect an effect of age or weather on departures. The decision by lean birds to reorient movement inland may reflect the suitability of the coastal stopover site for deposition of fuel stores and the motivation to seek food among more extensive forested habitat away from the barrier.
... Precipitation is avoided as it causes weight increase and heat loss (Richardson 1978). Although multiple orientation cues may be applied by long-distance migratory birds which fly throughout both day and night, and because they need to cope with the absence of some cues in certain latitudes (Moore 1980(Moore , 1987Gudmundsson and Sandberg 2000;Grönroos et al. 2010), it is easier to access light-relative cues, such as sun direction and skylight polarization when departing in clear sky. ...
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Background: Departure decisions and behaviors of migratory birds at stopover sites are expected to maximize fitness by trade-offs among avoiding predators, optimizing refueling (energy) capacity, and matching other life-history events. We predict that species with different body sizes and migratory destinations will exhibit different behaviors when departing from the same stopover site. We also predict that with strong time constraint at the final pre-breeding stopover site, departure decisions may be less sensitive to exogenous factors, such as wind condition, compared to other stopover or nonbreeding sites. Methods: We recorded migratory departures of four shorebird species, i.e. Eurasian Curlew (Numenius arquata), Bar-tailed Godwit (Limosa lapponica), Great Knot (Calidris tenuirostris), and Grey Plover (Pluvialis squatarola), at Yalujiang Estuary Wetlands in China, a final pre-breeding stopover site in the northern Yellow Sea, from 2011 to 2014. We compared flock sizes, departure time and departure directions between species, and investigated the effects of tide and weather conditions (rain and ground wind speed and direction) on the departure decision of shorebirds. Results: We found that larger species departed in smaller flocks and were more variable in daily departure time. Departure trajectory of all four species appeared to be influenced by coastal topography. With the east-west coastline and intertidal mudflat on the south, birds exhibited westward or eastward deflection from the shortest migratory routes. Bar-tailed Godwit was the only species that deviated to the southeast and did not climb over the land. Birds avoided departure during precipitation, while their departure was not related to ground wind benefit or tidal condition. Conclusions: Body size among species, which influences their vulnerability to predators, might be important in shaping shorebird departure strategies. Diverse departure directions could be the result of different wind use tactics in climbing stage. Narrow optimal time window of breeding might lead to reduced flexibility in departure date at a final pre-breeding site. Both endogenous and exogenous are important in shaping departure behaviors and decisions.
... Inexperienced birds must learn to use the sun compass; in homing pigeons, learning appears to take place after the bird is confronted with the need to home, likely spontaneously during extended exercise flights around the home loft. Moore (1987) demonstrated that the position of the setting sun functions as a source of directional information for night-migrating birds. Experimental evidence indicates that several species of migratory birds (but not all; see Wiltschko et al. 2008) are apparently able to perceive polarized light and use it as a cue for orientation. ...
Chapter
Migration is a complex behavior that offers a rich study system for evolutionary biologists, as it involves countless morphological and physiological adaptations for efficient movement, behavioral adaptations for optimal use of environmental factors such as winds and sensory cues for orientation, and cognitive adaptations for map learning, long-term memory, and social interaction.
... As a proxy for the departure intensity of migratory birds, we considered the migration intensity within the first two hours after sunset (Biebach et al., 2000). Previous studies report that most of the migrant birds depart during the first few hours after sunset (Åkesson et al., 1996;Moore, 1987;Sjöberg et al., 2017) and typically land before sunrise Moore, 1986). Bruderer and Liechti (1999) showed that main take-off starts 30 min to 1 h after sunset, while under favourable weather conditions peak densities are observed 1−2 hours after dusk. ...
Article
The spatial and temporal patterns of broad front bird migration are governed by the geospatial distributions of landmasses, mountain ranges, and weather conditions. These distributions interact with the birds' innate program during migration and are critical to successful migration. Hence, favourable environmental conditions for migration consequently lead to spatio-temporal concentrations, and the evolution of specific migratory flyways. Based on the aerodynamic properties of the ten most abundant nocturnal long-distance migrants, we developed a computational framework to simulate millions of individual trajectories across Europe using an agent-based simulation approach. We simulated a three-week period of autumn migration with a temporal resolution of 30 s. Departure conditions were derived from bird densities observed within the first two hours after sunset, which were extracted from the weather radar network. Individual itineraries were strongly influenced by the behavioural reactions to the environment such as the wind flow, coastlines and mountain ranges. Wind speed and direction are among the key factors that shape migration patterns such as reverse movements observed within some nights. Accumulation of migration was triggered by the combined effect of geographic barriers (coastlines, mountain ranges) and wind. The overall result of the simulation corresponds well with the large-scale pattern of bird migration intensities measured across the study area, for instance the high migration intensity observed between the Atlantic coast and the Pyrenees. Our model framework conjoins all important domains, such as the birds’ preconditions and behavioural scope, as well as the spatio-temporal dynamics of the environment. However, for the time being we cannot decide whether local discrepancies between model and data are due to environmental effects that are not yet captured by our simulation (i.e. effect of rain on migration intensity), or caused by differences in the sensitivity of the local weather radar systems. Due to the limited time period and lack of more accurate data for validation, our findings are preliminary. Further progress in data quality and hopefully better access of bird profiles from the continent-wide weather radar network will allow to improve model performance and implement a bird migration forecasting similar to weather forecasts. Apart from assessing conflicts between human activities and bird movements, this would greatly enhance our understanding of biomass flow on a large scale.
... Bird species are often excellent travellers and have been widely used as animal models for studying spatial orientation (Jouventin and Weimerskirch 1990;Wallraff 2005;Egevang et al. 2010;Tommasi et al. 2012;Gagliardo 2013;Mayer et al. 2013Mayer et al. , 2016Wilschko and Wiltschko 2017;Bingman 2018;Mouritsen 2018). To find the correct course when traveling long distances, birds use different types of information, such as the sun and star compasses, the Earth's magnetic field and the polarization pattern of the sky (Moore 1987;Ossenkopp and Barbeito 1978;Wiltschko and Wiltschko 2005). However, this information provides only the direction of the goal location and not its exact location. ...
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Birds have been widely used to study spatial orientation. However, since different birds rely on different types of visual information to find goal locations (such as spatial information from free-standing objects or local cues, i.e. characteristics of a goal location like color and shape), it is important to investigate this aspect in each model species. The aim of the present study was to clarify whether domestic chicks, a ground-living bird and a widely used model for the comparative study of spatial orientation, are able to reorient in relation to free-standing objects and if they preferentially follow local or spatial cues. Furthermore, we also investigated whether monocular eye occlusion influences the ability of chicks to use spatial or local cues. Chicks were trained and tested in a large circular arena with free-standing objects providing relational spatial information, to find food in one of the feeders. We found that dark-incubated male chicks were able to reorient in relation to distinct, free-standing landmarks (Experiment 1), but when local and spatial cues were put in conflict, chicks significantly preferred local cues over spatial cues (Experiment 3). Moreover, while the use of one eye system only was not sufficient to orient by spatial cues (Experiment 2), the preference for local over spatial cues was independent of monocular occlusion (Experiment 4). The results are discussed in relation to our general knowledge of spatial information processing in domestic chicks.
Article
Four species of nocturnal trans-saharan passerine migrants (Ficedula hypoleuca, Phylloscopus trochilus, Phoenicurus phoenicurus, Sylvia borin) were tested in Emlen funnels to assess the relative importance of the setting sun and stars in determining the nocturnal course. The experiments took place during spring migration at a Mediterranean stop-over site (Sardinia, Italy). The birds were caught during the day in mist-nets and kept in opaque plastic cages preventing them seeing outside, until the time of the experiment, on the same night; each bird was tested only once. Two experimental groups were exposed to the sunset, the first including the sight of the sun disc, the other after the sun had set. A third group was kept in the closed cages until the time of the test. A fourth group was directly tested in the funnels at sunset time, after the sun disc had set. The first three groups were tested in the funnels later, in complete darkness. Exposure and tests occurred under the same sky conditions, either clear or overcast. Under a clear sky, 10 out of 16 distributions are oriented in the expected migratory direction, or have a tendency towards it, irrespective of their exposure to the setting sun. This shows that the cues present at sunset are not necessary for the orientation of these birds, whose ability to orient correctly in the absence of any celestial cues is also confirmed by the results of the tests performed in overcast conditions with no previous exposure to visual cues. Unexpectedly, exposure to the setting sun with an overcast sky, followed by a test in the same overcast conditions lead to disorientation, suggesting that exposure to visual cues makes the birds look, at least at first, for other similar cues before turning to non-visual compass information.
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Most flocks consisted of 10-60 individuals, with the larger species occurring in larger flocks. When conspecifics were best "available' (during peak migration and at high tides) starting flocks were largest. During departure flocks sometimes joined others or split up, "stabilizing' between 30-60 birds. Starting flocks usually assembled from clusters into V-formations or echelons. About 4% of the flocks contained >1 species. In >90% of the departing flocks birds vocalized intensely. Flocks leaving the Banc d'Arguin reached heights of 1.5 km, with no signs of flocks levelling off. During this ascent birds usually headed into the wind with climb rates of 0.55-0.69 m s -1. Except under the rare condition of tailwinds, departing flocks showed no compensation for wind drift. A fixed orientation would make it easier for the birds to calculate wind drift effects between successive geographical fixes on which to base subsequent orientational adjustments. All species headed towards NNW, instead of NNE where the next stopover areas are found. This westerly orientation might represent an adaptation to correct for a continuous easterly drift at high altitude. Most flocks left before sunset, the departures showing a variable relationship to tide height. In the Dutch Wadden Sea, where cloud conditions vary, all witnessed departures occurred with a visible sun. -from Authors
Article
Cage experiments with various species of migrating passerines indicate two distinct phases in migratory orientation in which magnetic and celestial cues interact in different ways. The first phase during the premigratory period involves the conversion of genetically coded information into an actual compass course. Celestial rotation and the geomagnetic field serve as external references. Celestial rotation alone, mediated by rotating stars at night or by the changing pattern of polarized light during the day, provides a reference direction away from its center, corresponding to geographic South. Population-specific deviations from this reference direction appear to be coded only with respect to the magnetic field. Both cue systems interact to produce the population-specific migratory course. In the role of providing the reference direction, celestial rotation dominates over the magnetic field during this first premigratory phase. Magnetic South, however, can also serve as reference direction when celestial cues are not available. The second phase involves orientation during migration, once the course is set and the birds have left their breeding area. Migrants en route have several options to locate this course, namely a star compass, sunset cues and the magnetic field, and they seem to make use of them all. During this phase, however, the magnetic compass dominates over celestial cues, as indicated by cue-conflict experiments. In case of conflict, the directional significance of stellar cues and sunset cues is adjusted to be in agreement with the ambient magnetic field. Migratory orientation is thus based on an integrated system of celestial and magnetic cues that reverse their dominant role between the premigratory phase and actual migration. The reasons may lie in the changes in the sky and in the magnetic field migrants experience when travelling from higher to lower latitudes towards their winter quarters.
Chapter
I claim that the currently widely accepted concept that migratory take-offs occur within a narrow time window soon after the sunset is not generally correct. Observation and radio-tracking data show that in a number of songbird migrants departures occur well into the night. The body mass and fuel stores of departing migrants vary broadly; a substantial proportion of birds take off with low fuel stores that do not allow them to fly throughout the night. Some migrants perform reverse migration during the night; however, others probably make short flights in the seasonally appropriate migratory direction. Our field data on the body mass loss during the flights support the recent wind tunnel measurements that suggest that migratory flight in long-distance migrants is significantly less expensive than was hitherto assumed: some 6–7 times basal metabolic rate (BMR), not 10–12 times BMR.
Article
Time of departure and landing of nocturnal migrants are of great importance for understanding migratory strategy used by birds. It allows us to estimate flying time and hence the distance that migrants cover during a single night. In this paper, I studied the temporal schedule of nocturnal departures of European robins during spring migration. The study was done on the Courish Spit on the Baltic Sea in 1998–2003 by retrapping 51 ringed birds in high mist nets during nocturnal migratory departure. Take-offs of individual birds occurred between the first and tenth hour after sunset (median 176 min after sunset). Departure time was not related to fuel stores at arrival and departure, stopover duration and progress of the season. The results suggest that one reason for temporal variation in take-off time was differential response of European robins with high and low motivation to depart to such triggers as air pressure and its trend. If these parameters reach a certain minimum threshold shortly before sunset, robins with a high migratory motivation take off in the beginning of the night. When air pressure or its trend reaches a maximum, it may trigger to take off later during the night birds with lower initial motivation for departure, including those that have low refuelling efficiency. In regulation of timing of take-offs of robins, an important role is also played by their individual endogenous circadian rhythm of activity which is related to the environment in a complex way.
Article
The collision of migrating birds with human-built structures and windows is a world-wide problem that results in the mortality of millions of birds each year in North America alone. Birds killed or injured at such structures are due to two main factors. The first of these is the lighting of structures at night, which “traps” many species of nocturnal migrants. The second factor contributing to the hazard is the presence of windows, which birds in flight either cannot detect, or misinterpret. In combination, these two factors result in a high level of direct anthropogenic (human-caused) mortality. Bird mortality at human-built structures receives relatively little public attention, but structural hazards are actually responsible for more bird kills than higher profile catastrophes such as oil spills. The purpose of this report is to summarize what is currently known about migratory bird collisions, to investigate the seriousness of the threat, to present data on migratory bird mortality in central Toronto, and finally to make preliminary recommendations on how to help eliminate the problem.
Article
By computer simulations of different flight routes for spring migrating Knots Calidris canutus, potential orientational 'danger zones' were revealed in the equator area and north of the Arctic circle. An evaluation of the availability of directional information during migratory flights demonstrated the potential hazards of crossing these danger zones. Investigations of the directional consequences of sun compass orientation based upon an internal clock in phase with local time at the departure site showed that such a mechanism would entail serious losses in distance and time. A simple compensating rule based on the rate of change in sun altitude was found to work well both at polar latitudes and in the equator area. The magnetic compass will become inoperative close to the magnetic equator and the poles, thus restricting the availability of directional information. Orientation by stellar cues will become impossible during spring and summer at arctic latitudes. Simulations of potential migratory routes also enabled examination of the required precision of compass systems and it was indicated that necessary orientation accuracy requires more than one directional selection per long-distance flight to ensure survival. It is concluded that computer simulations of flight tracks, based on ringing recoveries, visual observations, radar studies and satellite telemetry may be a powerful tool helping to identify problems which require future experimental attention.
Article
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The time of departure of nocturnal passerine migrants was studied by radiotelemetry and tracking radar in South Sweden in spring and autumn. Our objective was to analyse the time of flight initiation in relation to celestial orientation cues during the twilight period at dusk. The telemetry and radar results were compared with data on nocturnal flight departures reported in the literature. There was a considerable variation in time of flight initiation throughout the twilight period. A large fraction of the birds (mostly thrushes) departed mainly during the civil and nautical twilight periods, with a sun elevation between 0° and -12°, while several migrants did not leave until much later in the night. This variation in time of departures speaks against the hypothesis that departure time is connected with a critical skylight situation with respect to the visual accessibility of celestial orientation cues. Characteristic times of departure, as reflected by the associated elevations of the sun below the horizon, seem to differ between different passerine species, latitudes and times of migratory season.
Chapter
In this chapter I emphasize that qualitative stopover ecology should be studied in a close association with the stopover behaviour, primarily with habitat use and selection by migrants. During stopovers, migrating songbirds are confronted with unfamiliar habitats, whereas they need to forage efficiently and refuel quickly. The habitat use of migrants is non-random, and I analyse the process of habitat selection and use on the basis of capture data and of radio tracking results of passerine migrants at stopovers. I discuss hierarchical decision making process when ceasing migratory flights and selecting stopover habitat, and whether this process is age-related. The importance of broader landscape context for stopovers, apart from the immediate habitat structure, is emphasized.
Chapter
Location is crucial in the life of an animal; in this chapter we cover the mechanisms used by animals to find food, shelter, mates, and the right microclimate. Animals may find good habitat by moving randomly, but more often movements are oriented to external stimuli. After determining the location of a stimulus, an animal moves toward that stimulus if it is attractive, or away from it if it is repulsive. Compass orientation gives animals the ability to move at an angle with respect to the location of the sun, moon, stars, or the earth’s magnetic field; homing and migration typically depend on compass orientation. In addition, path integration is important to many animals. When using path integration, an animal calculates an efficient route to its nest or den based on its outward journey. Migration allows animals to find seasonally appropriate habitats but requires complex navigation that can span thousands of kilometers.
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Migratory birds can use a variety of environmental cues for orientation. A primary calibration between the celestial and magnetic compasses seems to be fundamental prior to a bird's first autumn migration. Releasing hand-raised or rescued young birds back into the wild might therefore be a problem because they might not have established a functional orientation system during their first calendar year. Here, we test whether hand-raised European robins that did not develop any functional compass before or during their first autumn migration could relearn to orient if they were exposed to natural celestial cues during the subsequent winter and spring. When tested in the geomagnetic field without access to celestial cues, these birds could orient in their species-specific spring migratory direction. In contrast, control birds that were deprived of any natural celestial cues throughout remained unable to orient. Our experiments suggest that European robins are still capable of establishing a functional orientation system after their first autumn. Although the external reference remains speculative, most likely, natural celestial cues enabled our birds to calibrate their magnetic compass. Our data suggest that avian compass systems are more flexible than previously believed and have implications for the release of hand-reared migratory birds.
Chapter
The study of animal movements in the environment is a journey in itself, starting with the integration of simple sensory inputs and basic movements. For many animals, the ability to discover and orient to food, shelter, and mates is all that is needed for evolutionary success. Kineses, or undirected movements, and taxes, or directed movements, provide the basic mechanisms by which many animals find their appropriate habitats. The discovery of the direction or location of resources is often the key to navigate to those resources. Search behavior typically begins with undirected or looping movements and, as cues are gained from the environment, shifts to goal-directed movements. Sensory systems, such as stereoptic vision, are required for goal-oriented movements. For some animals, navigational needs extend far beyond these abilities. Navigation sometimes relies primarily on genetically sourced information, the product of generations of evolutionary trial and error. In other cases, learning plays a larger part in navigation, giving the animal the flexibility to accommodate its current location and to exploit information obtained from other population members. Path integration and compasses are often used in homing. Finding home requires using sophisticated mechanisms such as path integration, landmarks, and snapshot memory.
Chapter
Birds have evolved a mobile lifestyle in which vision is of major importance when controlling movements, avoiding predators, finding food and selecting mates. Birds have extraordinary colour vision and have been suggested to perceive the linear polarisation of light. Behavioural experiments support this idea, but still the exact physiological mechanism involved is not known. The twilight period, when the sun is near the horizon at sunrise and sunset, is of crucial importance for migrating birds. At this time millions of songbirds initiate migration when the degree of skylight polarisation is the highest and all compass cues are visible in a short range of time. The biological compasses are based on information from the stars, the sun and the related pattern of skylight polarisation, as well as the geomagnetic field, and may be recalibrated relative to each other. The celestial polarisation pattern near the horizon has been shown to be used in the recalibration of the magnetic compass, but conflicting results have been obtained in experiments with different bird species. For the future we should understand the physiological mechanisms of avian polarisation vision and investigate the interrelationship and calibrations between the different compasses, including the one based on the pattern of skylight polarisation. A conditioning paradigm may be fruitful, but the risk of introducing optical artefacts needs to be minimised in behavioural experiments, as well as in cage experiments with migratory birds.
Article
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This perspective on the stopover biology of migratory landbirds is organized around the response of migrants to challenges that can arise during stopover and how events during stopover are interconnected with other phases of the annual cycle. Landbird migrants seldom move nonstop from origin to destination, rather they stopover periodically, sometimes for a few hours, sometimes days. How well migrants meet challenges that arise during passage in a timely manner influences the success of their migration; a successful migration is measured in terms of survival and reproductive performance over the annual cycle. What transpires during migration is not isolated from other phases of the migrant's annual cycle. Events taking place in one phase carry over to affect an individual's biology in other phases, including linkage between breeding and wintering phases and the intervening migratory phase.
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Pied flycatchers were tested twice at sunset under a clear sky, a clear sky with moon, and under artificial overcast conditions. The mean spring direction was W‐NW, with a higher westward bias than in previous tests performed in complete darkness. The activity of single tests was high but their directionality was low, with an inverse correlation between activity and directionality. Under clear skies (with or without moon) the distribution of birds performing a test for the first or second time was not different, with high consistency of individual birds, while in overcast conditions second tests shifted southwards, seemingly a phototactic reaction. Under clear skies (with and without moon), in the second test individual vector lengths increased and there was no correlation between individual activity and vector value, indicating an improvement of the perform ance; in contrast, in second overcast tests vectors tended to decrease and the negative correlation activity/vector was significant. So, although the mean direction was not different from the clear sky, the individual performance was worse in overcast conditions, probably due to a decrease in motivation to migrate during prolonged overcast conditions.
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Many vagrant landbirds have occurred on Brier, Sable, and Seal islands, Nova Scotia and have produced first records for Nova Scotia or Canada. In counts made on these islands for more than a decade, individuals of western species were relatively more common than in similar counts on islands off northeastern Florida in spring and North Carolina in fall. Western vagrants were more frequent, but much less diverse, on the Florida islands in fall. Recent check-lists substantiate the conclusion that vagrant species from remote ranges are more diverse on the Nova ScotJan islands than in other localities in eastern North America. The diversity and high incidence of vagrants may be related partly to the convergence of continental windstreams on Nova Scotia during migration seasons. Regional species, however, evidently avoid Sable Island, which is farthest offshore, whereas vagrants from remote ranges are relatively more common there than on the other two islands. This suggests that navigational error by "lost" vagrants is para-mount. Weather patterns are examined in association with the earliest vagrants of each season, as these vagrants may have come most rapidly and directly. Southern vagrants may fly past their normal ranges in spring, unassisted by winds. Western species in spring are probably not generally deflected from their normal routes by winds. Western Dendroica spp. in spring in the east may come because of mirror-image disorientation. Southern vagrants are proportionately as common in fall as in spring, and individuals may or may not be wind-assisted in their reverse migrations. Western vagrants are relatively more common in fall than in spring. Some may come directly down-wind on fall westerlies, but their relatively high incidence in Nova Scotia may result from their being swept up the east coast by prevailing southwesterlies. Turn-of-the-century records from Sable Island suggest that vagrancy may have increased among species that have expanded their populations or ranges, especially those of disturbed habitats.
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Nocturnal autumn passerine migrants are frequently drifted by wind or carried by downwind flight from New England to offshore islands. Most individuals in these flights are immatures. Many recover the mainland via reoriented northward flights. During fall 1972, I made field observations and performed orientation cage tests on nocturnal migrants on Block Island, Rhode Island. Migrants reached the island during four southward movements in following winds. Migration was very light or absent on most other nights. Reoriented diurnal flights left from the island immediately after the arrival of a large nocturnal migration. Northward movements occurred on two nights following daytime reoriented flights. These nocturnal movements are tentatively interpreted as constituting reoriented flights. About 27% of the 79 individuals tested in orientation cages showed significantly directional nocturnal activity. About two-thirds of these oriented basically northwestward, corresponding to the reoriented flight of free-flying birds. These and similar reorientations of immature birds can be explained by a simple compass reorientation in response to wind drift over the ocean without invoking complicated navigation mechanisms. The remaining individuals oriented toward the southeast. Eleven birds exhibited a significant eastward orientation during the first few hours after dawn; this is interpreted as a positive phototaxis. Three of four Blackpoll Warblers oriented southward. No correlation existed between the quantity of Zugunruhe and the amount of subcutaneous fat, but fatter birds were significantly more likely to show oriented nocturnal activity.
Article
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Field observations reveal that night-migrating passerines sometimes fly during early morning hours. Functional consideration of daytime flights have overshadowed proximate ques- tions regarding the sensory basis of daytime orientation. I conducted a series of early morning cage-orientation experiments in spring with four species of migrating warblers (Parulinae) on the north coast of the Gulf of Mexico. Whereas birds were poorly oriented on overcast mornings, the headings for three species were oriented in a seasonally appropriate direction when individuals were tested on clear mornings with the horizon glow from the rising sun visible. Moreover, the direction of activity was influenced in a predictable way by manipulating the pattern of skylight polarization. Skylight at sunrise appears to function as a source of directional information for morning flights of migrating warblers. This interpretation is consistent with field observations of the orientation behavior of free-flying migrants, and reinforces speculation that solar-related in- formation is involved in the orientation of ongoing migratory flight, or redetermined orientation following nighttime displacements. In any case, the responsiveness of several species in a cage situation may allow controlled experiments on the question of course correction and redetermined migratory flight.
Article
Data were analysed for fall 1968 to spring 1978. Species were divided into 12 subgroups to reflect seasonal and geographical distributions on the mainland. Abundances of the various subgroups were directly related to the proximity to the island of their breeding and wintering ranges and normal migration routes. For most subgroups, abundances were greater in fall than in spring. The great majority of fall individuals were hatching-year birds, and many, if not most, spring individuals were probably 2nd-year birds. Highly significant year-to-year variations occurred in the number of individuals that arrived on the Farallones and in the proportion of individuals in the various subgroups, despite the fact that the number of species and proportion of species in the various subgroups remained statistically invariant from year to year. The annual variations in abundance of Farallon landbirds were probably caused by variations in the number of young produced each summer, by long-range weather patterns acting over large areas of the continent, and by variations in local weather conditions.-from Author
Article
By considering the weights of the heaviest members of a population of Yellow wagtails about to emigrate from northern Nigeria, estimates of the mean weight of each sex at take-off are derived. These are used to predict changes in weight and flight speed during a non-stop trans-Saharan flight. For most birds the duration of this flight will be 60–70 h, enabling them just to attain the northern side of the desert. Some birds may resort to partial dehydration to achieve this, but extensive dehydration is not apparent for most birds caught towards the end of the flight. It is apparently essential that favourable winds occur during the flight; even slight head winds are likely to produce heavy mortalities. Because Yellow wagtails depart on a trans-Saharan crossing with just enough reserves to achieve this, adverse winds, difficult conditions immediately north or south of the desert, or any avoidable extension of the flight are all likely to impose considerable selection pressures. Therefore, probably all Yellow wagtails move gradually northward in Nigeria, to the southern margins of the desert, where they may still obtain adequate food, and there await favourable meteorological conditions before embarking.
Chapter
The daytime flights of nocturnal migrants are examined in relation to the principal axes of migration (P.A.M.) based on calm nights during spring and fall in northwest South Carolina. When nocturnal flights were to the east or west of the calm night directions (30° spring, 232° fall), flights occurred the next morning basically perpendicular to these directions. Because most migrants were displaced east of their P.A.M. in spring and fall, most daytime flights were directed toward the NW, but when nocturnal flights were west of the P.A.M., flights the next morning were to the SE. The daytime redetermined migrations permit the displaced migrants to regain their migratory route by flying the shortest distance.
Chapter
A new technique for studying the orientation of free-flying migrants following experimental manipulation is described. Birds are taken aloft, rereleased at migratory altitudes, and their flight departures accurately plotted by tracking radar. Experiments on the relative importance of different cue systems in the spring migratory orientation of White-throated Sparrows suggested: (1) Sparrows rapidly selected appropriate, straight, migratory tracks when released under clear night skies or during periods of sunset glow. (2) Under total overcast sparrows still were able to select meaningful headings although with much less accuracy. They frequently adopted a sinusoidal, “zigzag” flight strategy under these conditions. (3) Landmark cues of the Atlantic coastline were used minimally if at all by the migrating sparrows.
Chapter
Orientation tests in the afternoon, at night and in the morning show that blackcaps and robins select their night migration direction before nightfall. But this process does not only depend on light. In a magnetic field with a reduced total intensity during the night (0.05 Gauss, mN - - -, 89° Incl.) the birds are able to select their migration direction. But they are totally disorientated in a magnetic field with a changing horizontal component by a day-night rhythm. In an analogous test arrangement with stationary stars, robins can maintain the migration direction. Under laboratory conditions an artificial sunset cannot be fixed as an orientation cue.
Chapter
Experienced non-resident homing pigeons that are housed in deflector lofts for as little as seven days (i.e. “short-term” residence) show upon release a deflection of initial orientation in the direction predicted by the olfactory theory of homing. This effect, however, is confined to the summer months, disappearing after August 31. Altered deflector panels, which reverse the direction in which reflected light cues are rotated while leaving wind rotation unchanged, reverse the deflection of initial orientation. A model is proposed to explain these results in which the deflector lofts rotate a light-based reference system (derived from polarized light patterns present at sunrise and sunset) which is then used by the birds to calibrate their sun compass.
Article
Orientation exhibited by animals that presumably cannot see the sun as a discrete source because of clouds or because of experimental interference with detailed vision, is often considered to be indicative of nonvisual cues. However, the remaining anisotropic radiance distribution may also be a directional cue. A vectorial parameter, the directivity D, is proposed for the directional quantification of a radiance field. Its computation resembles that of the test statistic of the Rayleigh test. Theoretically D ranges between 0 in an isotropic radiance field, and 1 in a parallel beam. Various examples are given of orientation in an anisotropic radiance field.
Article
Observations from a network of radars on ships and on islands in the western North Atlantic Ocean and on the eastern coast of North America suggest that successful autumnal migrants moving between North and South America utilize simple compass orientation and do not require bicoordinate navigation. Successful migrants maintained a southeast heading from the North American coast to the Caribbean, Unsuccessful migrants became disoriented over the Atlantic. Visual observations suggest that successful migratory behavior is restricted to certain taxonomic groups of passerines.
Chapter
Radar tracks of individual birds migrating on overcast nights were compared with a similar set of tracks obtained under clear skies. I examined the accuracy of orientation within and among tracks, flight altitude and changes in altitude, the effects of flying within clouds, and the effects of continuous overcast of several days’ duration. When birds were flying below the cloud cover there were essentially no discernible effects. Their flight was as straight, level, and fast as comparable birds flying on clear nights. Likewise, the spread in headings among birds was similar. On each of four nights when birds appeared to be flying within or between cloud layers, headings were random and the individual tracks were slightly, but significantly less straight than when birds were flying beneath the clouds. Uninterrupted overcast of several days’ duration did not result in random headings so long as the birds were flying below the cloud ceiling.
Chapter
Although the distance of movement with respect to age has been emphasized in many publications on avian dispersal and migration, comparatively little has been said about the orientation of such movements with respect to age. In this paper the age-dependent orientation of birds during dispersal and migration is examined. The results suggest that much of the interindividual variation in the orientation behavior of birds may be explained by age-specific effects. Adults and young may have different nonbreeding areas, and the orientation of movements to these areas may differ depending on the ages of the birds. Young are also more strongly influenced by disruptive orientational cues in the environment (e.g., lights from cities), and responses to these cues may take young, inexperienced birds away from the normal migratory pathways of the adults. Future studies of migratory orientation will have to consider age-specific effects, if a correct biological interpretation of the results is to be achieved.
Article
The importance of sunset as a fall orientation cue in the white-throated sparrow {Zonotrkhia albkollis) was examined under clear skies, cloudy skies, and after sparrows were denied sunset exposure. Sparrows tested during sunset under clear skies exhibited a significant southerly orientation. Those tested under cloudy skies exhibited a more dispersed orientation which was not significant. Sparrows denied sunset exposure but allowed to view stellar cues, exhibited a marked decrease in activity and no directional preference. Results suggest sunset is a primary visual cue functioning in selection of direction prior to nocturnal fall migration.
Article
Relationships of short-term weather to daily migration intensity are reviewed, with sections for each weather variable, for waterfowl, shorebirds and hawks, and for reverse migration. Selecting factors, methodology, hypotheses and results to date are summarized, generalizations about migration-weather relationships are extracted from the pooled results, and high-priority research topics are identified. Ultimate factors responsible for present responses are thought to include aspects of weather en route and a the take-off point and destination. Proximate factors affecting the probability of take-off may include variables useful in predicting, as well as those that measure, aspects of weather with selective significance. Causative and coincidental relationships remain difficult to separate, and at least a few birds migrate in almost any weather conditions. However, maximum numbers migrate with fair weather, with tailwinds and with temperature, pressure and humidity conditions that accompany tailwinds. Correlations with weather differ among populations with different flight directions. General patterns of responses appear to be modified by special selecting factors that apply to certain groups - soaring hawks tend to fly on days with strong updrafts, landbirds migrating along coasts prefer onshore to offshore winds, and transients in unsuitable habitats seem less likely to wait for favourable travelling conditions. /// дан обзор взаимосвязи погодных усповий и иитенсивности суточных миграпий с раздепами, посвященными анализу впияния отдельных погодных факторов на миграции водоплавающих, береговых и хищных птиц и на обратные мнграции. Обобщены сепеютирукщие факторы, методопогия, гипотезы и результаты, имеющиеся в настоящее время, сдепаны выводы о взаимосвязях миграции-погода, на основании имеющихся материапов определены генералвные направюения иччледований. Предполагается, что ультимативные фактопы, определяющие эти отношения, включают характер погоды по маршруту миграции, а также в точках отлета и назначения. Непосредственное влияние на отлет могут оказывать различные аспекты погоды, которые можно измерить ипи предсказать: отдельные факторы имеюь различную значимость. Причинные и сопутсвующие связи трудно разделить и, по крайней мере, некоторые птицы мигрируют при погодных условиях. Однако, максимальное количество птиц мигрирует при хорошей погоде, попутном ветре и соответствующих температурах, атмосферном давлении и влажности. Основной характер взаимосвязей может модифицироваться специальными фактояами, которые имеюь значение для некоторых хищников, парящих группами и предпочитающих дни с сильными вертикальными токами, а также у сухопутных птиц, мигрирующих вдопь берега и предпочитающих ветер с моря береговому бризу, и наконец, у перелетных птиц, оказывающихчя на пролете в неподходящих местообитаниях, которые не дожидаются наиболее благоприятных условий для перелета.
Article
Modified "Emlen funnels" are used for continuous electronic recording of the directional distribution of Zugunruhe movements in night-migrating passerines. An artificial stellar sky as well as an artificial magnetic field can be set in any direction and can rotate slowly in the horizontal plane. Birds may follow this rotation within their funnels thus demonstrating that their preferred direction is controlled by one of the cues presented. The birds were clearly oriented in relation to the artificial stars, but it has still to be clarified by which criteria they decide for a particular direction. In contrast, responses to the magnetic field could not be observed or, in some experiments, were indicated quite weakly. /// Модификация воронки элмена использовалась для длительной электронной регистрации направленного распространения движения " Zugunruhe" у ночных воробьиных птиц. Искусственные звездное небо н магнитное поле помешались на некотором насравлении и начинали медленно вращать в горизонтальной плоскости. Птицы способны следовать за этими вращениями внутри своих воронок, показывая тем самьм, что предпочитаемое ими направление определяется одним из имеющихся ориентиров. Птицы очевидно ориентируются в отношении искусственных звезд, но попрежнему остается необъячненньм, какими критериями они пользуются при ныборе определенного направления. В противоположность этому реакция на магнитное поле не наблодалась или в некотопых опытах была очень слабой.
Article
The orientation behavior of robins Erithacus rubecula, garden warblers Sylvia borin and whitethroats S. communis was compared in the presence and absence of stars, to examine the influence of stars and magnetic field on the accuracy of orientation. All data were recorded in an electromechanically registrering cage with eight radially positioned perches. In the natural geomagnetic field or a field of comparable intensity (0.46 Gauss), the vector length r<sub> m</sub> of the test series, calculated from the headings of the individual test nights, was similar in the presence and absence of stars, a slight increase under stars being observed only for the robin. The concentration a<sub> i</sub> around the selected direction was, however, strongly increased by the stars; artificial and natural ones having the same effect. In a reduced magnetic field (0.34 Gauss), no preferred heading direction was found, but again the concentration a<sub> i</sub> was greatly increased by the presence of artificial stars. Thus selection and maintenance of a direction are two separate, independent processes: the determination of the migratory direction depends mainly on the magnetic field and is only slightly affected by stars in the robin, whereas the maintenance of a direction strongly depends on the presence of stars. Since meaningless headings were maintained with high concentration in the reduced magnetic field it appears that under stars only one independent directional selection normally is performed per night. /// Сравнивали характер ориентации Erithacus rubecula,Sylvia borin и S. communis при наличии и отсутствии звезд для выяснения влияния звезд и магнитного поля на точность ориентации. Все данные получены в клетках с злектромагнитньм счетчиком и с 8-ю радиально расположенньми жердочками. В естественном геомагнитном поле или в поле близкого напряжения (0,46 гауссов) длина вектора r<sub> m</sub> опытных серий, пассчитанная при направлениям в отделнье ночи, когда проводились опьпы, была схолной при наличии и отсутствии звезд, причем, слабое увеличение ее при звездах наблюдалось лишь у E. rubecula. Концентрация a<sub> i</sub> вокруг выбранного направления однако, резко повьшалась при звездах. Искусственные и естественные поля давали одинаковый ьффект. В более слабом магнитном поле (0,34 гаусса) установлено отсутствие основного напавления, но концентрация a<sub> i</sub> и здесь сильно увеличивалась в присутствии искусственных звезд. Таким обпазом выбор и поддержание направления - два отдельных независимых процесса: опрехеление направления миграции зависит в основном от магнитного поля н лишь в небольшой степени - от звезд. у E. rubecula сохранение направления в большей степени зависит от наличия звезд, так как случайные направления часто сохраняются в ослабленном магнитном поле. Предполагается, что при звездах в норме имеет место выбор лишь одного независимого направления в течение ночи.
Article
Nocturnal thrush migration was studied during three springs (20 March-30 April) and autumns (20 September-10 November) using an L-band, high-power radar station in southern Skåne. Both spring and autumn migration were divisible into two main types, i.e. ENE-(median 065°) and N-migration (001°) in spring and SE-(139°) and SSW-migration (207°) in autumn. ENE- and SE-movements dominated. The occurrence of these migration types was closely associated with tailwinds. Within each migration type, no influence of wind on flight direction was found, and the birds compensated completely for drift. Migration started on an average 32 min after sunset, and the peak of activity generally was at about 2 h after sunset. Migration intensity during spring showed significant positive correlations to SW- and W-winds, warm front passages and decreasing barometric pressure, and during autumn to winds from the sector NW to NE, cold front passages, sinking humidity and temperature, good visibility and little cloud cover. Discriminant analysis demonstrated that the predicted ground speed of the birds was the variable of overwhelming importance to account for night-to-night variation of migratory activity; high activity occurred on nights with high ground speeds. Factor analysis produced indications that the birds responded to single weather variables and not to general weather situations as such. /// Исследовали ночные миграции дроздов в течение 3-х весенних периодов (20 марта-30 апреля) и осенью (20 сентября-10 ноября) с применением L-колец и сильных радарных установок в южной Сконе. Весной и осенью выделеляются миграции двух типов - в северо-восточном-восточном на 0,65° и северном - 001° направлениях в весенний период и юго-восточном - 139° и северо-северо-западном - 207° направлениях - осенью. Преобладают миграции на северо-восток-восток и юго-восток. Направления этих мрграций тесно связаны с попутными ветрами. В пределах каждого типа ветер не влиял на направление полета, и птицы полностью компенсировали дрифт. Миграция в среднем начиналась через 32 мин. после захода солнца, а пик активности наблюдался в основном через 2 часа после захода. В интенсивности миграции в течение весны установлена положитальная корреляция с югозападными и западными ветрами, наступлением теплого фронта и понижением атмосферного давления, а осенняя миграция коррелирует в ветрами из северо-западного и северо-восточного секторов, с холодным фронтом, снижением влажности и температуры, хорошей видимостью и малой облачностью. Дискриминационный анализ показал, что влияние предсиказанной скорости движения земли на птиц колеблется за счет суточных колебаний активности миграций: высокая активность наблюдалась в ночи с высокой скоростью движения земли. Факторный анализ показал, что птицы чувствительны к отдельным колебаниям погоды, а не к общей погодной ситуации.
Article
To investigate the relative importance of stellar and magnetic cues for the compass orientation of night migrating birds, 45 European robins (Erithacus rubecula) were tested in automatically registering cages with view of the clear natural night sky. One group was tested in the natural local geomagnetic field, the other group in a field pointing to 120° ESE; birds from both groups were additionally tested in a magnetic field the horizontal component of which was compensated. The observed orientation behavior leads to the conclusion that star compass and magnetic compass are not independent, but that they are interlinked in the way that the star compass is established by information from the magnetic compass.
Article
Drei Sperbergrasmücken (Sylvia nisoria) und ein Neuntöter (Lanius collurio), also nächtlich ziehende Vögel, wurden in einer 6-Wahlanordnung auf eine bestimmte Kompassrichtung dressiert. Die Übungen wurden zu einer bestimmten Tageszeit vorgenommen. Prüfungen zu einer Tageszeit, in der der Sonnenstand von dem der Dressur-zeit abweicht (bis zu 150°), zeigen als Wahlergebnis vorwiegend die richtige Kompassrichtung. Annähernd gleichwertige Ergebnisse erhält man, wenn die Sonne durch eine künstliche Lichtquelle ersetzt wird, die aus gleichbleibender Richtung strahlt, deren Höhe aber dem natürlichen Sonnenstand angeglichen wurde. Sperbergrasmücke und Neuntöter verfügen also ebenso wie Star und Brieftaube über einen Orientierungsmechanismus, in welchem der Sonnenstand unter Berücksichtigung der Tageszeit die entscheidende Komponente zum Auffinden einer bestimmten Himmelsrichtung bei Tage ist.
Article
Migrant white-throated sparrows (Zonotrichia albicollis) were released from boxes carried aloft by balloon and tracked with radar. All birds were released on clear nights when winds were light and opposed to the normal migration direction for the season. Birds were treated in one of two ways: Lens birds were fitted with frosted lenses prior to release; No Lens birds were released without lenses. No Lens birds that engaged in straight and level flight generally headed in the predicted migratory direction and as a group were not oriented with respect to wind direction. Lens birds did not head in the predicted migratory direction, but instead oriented downwind. This orientation behaviour is consistent with the relationship of orientation cues inferred from the field observations described in part I of this paper. The data show that flying birds deprived of all detailed form vision can determine wind direction.
Article
Tracking radar and visual observation techniques were used to observe the orientation of free-flying passerine nocturnal migrants in situations in which potentially usable directional cues were absent or gave conflicting information. When migrants had seen the sun near the time of sunset and/or the stars, they oriented in appropriate migratory directions even when winds were opposed. Under solid overcast skies that prevented a view of both sun and stars, the birds headed downwind in opposing winds and thus moved in seasonally inappropriate directions. The data point to the primacy of visual cues over wind direction, with either sun or stars being sufficient to allow the birds to determine the appropriate migration direction.
Article
Results clearly implicate the setting sun as a critical source of directional information in the migratory orientation of the savannah sparrow, Passerculus sandwichensis. Savannah sparrows allowed a view of both sunset and stars displayed oriented behaviour in biologically meaningful directions during spring and fall seasons. When the same individuals were denied a view of sunset, and tested under the stars alone, disorientation characterized their behaviour. Furthermore, birds allowed a view of sunset, but tested under ‘overcast’ night skies (no stars visible), displayed well-oriented behaviour indicating the sufficiency of sunset. Experiments in which the migrant's internal chronometer was shifted suggested a fixed-angle (menotactic) response to the sunset cue rather than a time-compensating compass mechanism. I believe stars are valuable to this migrant as celestial reference points. Orientational information gained at the time of sunset is transferred to stars on a nightly basis. The relationship between solar and stellar cues is apparently hierarchical in the savannah sparrow. Information necessary to select the appropriate migratory direction is gained from the primary cue, the setting sun, while maintenance of that heading is dependent on a secondary cue, probably the stars.
Birds can make use of the geomagnetic field, of the sun and the stars to find geographic directions. The magnetic compass represents an innate mechanism based on the birds' ability to perceive the earth's magnetic field while the sun compass and the star compass are learned. In the establishment of the latter, celestial rotation as well as the magnetic field have been shown to be involved. The operation modes of these compass systems and also the role of other factors known to affect birds' directional selections (e.g. sunset point, landmarks, wind etc.) are described and discussed.
Article
Among the many animals that perform long distance migration, such as butterflies, fish, marine-turtles and marine mammals, birds represent the most impressive performers. They are also the best-studied group, with many more details known of their migratory accomplishments and the factors involved in their orientation, than in other groups. The main question of how birds actually do navigate, however, remains unanswered. In this article Professor Schmidt-Koenig reviews the available evidence and suggests that a multiplicity of cues and strategies may be involved.
Article
In this paper we describe fall nocturnal migration at three localities in eastern New York, one adjacent to the Hudson River, the other two 30 km to the west in a topographically more uniform area. Migrants at both study areas moved southwest in winds not out of the west and were, therefore, seemingly unaffected by the river. In west winds, however, birds away from the river moved south-southeast whereas those in the vicinity of the river flew a track west of south paralleling the river. In addition, a relative increase in the number of migrants along the river compared to away was observed in west winds as birds presumably became concentrated near the river. We conclude that on most autumn nights migrants passing through this area have a preferred track direction toward the southwest and in strong winds from the west and northwest they are drifted. Upon reaching the vicinity of the Hudson River, some birds alter their headings yielding a track direction that closely parallels the river resulting in at least a partial compensation for wind drift. No alternative hypothesis is consistent with all the data.
Article
SUMMARY 1. Although many invertebrate animals orient by means of ultraviolet sky- light polarization patterns, existing measurements of these patterns are inadequate for full analysis of the biologically relevant information available from the sky. To fill this gap we have used a precision scanning polarimeter to measure simultaneously the intensity, degree, and direction of vibration (^-vector orientation) of polarized light at 50 intervals over the sky. The resulting sky maps were constructed for u.v. (350 nm) and visible wavelengths (500 and 650 nm) under a variety of atmospheric conditions. 2. Our measurements confirmed that the patterns of radiance and degree of polarization of skylight are highly variable and hence unreliable as orienta- tion cues; but patterns of E-vector orientation are relatively stable and predictable over most of the sky under all but very hazy or overcast conditions. 3. The observed E-vector patterns correspond more closely to predictions based on first order (Rayleigh) scattering at 650 and 500 nm than at 350 nm. This is true both in terms of absolute accuracy and the proportion of the sky with relatively' correct' information. Yet most insects respond to polarization patterns only at u.v. wavelengths. This apparent paradox can perhaps be re- solved by assuming that there is no great selective advantage for any par- ticular wavelength when large areas of blue sky are visible, but that under special and difficult conditions ultraviolet has advantages over longer wave- lengths. Measurements under partially cloud-covered sky, for instance, or under extensive vegetation, show that both spuriously polarized and un- polarized light resulting from reflexions present more troublesome inter- ference at longer wavelengths than in the u.v. 4. The accuracy of orientation achieved by dancing honey bees appears to be greater than can readily be accounted for by assuming that they use a strictly geometrical or analytical processing system for their orientation to polarized light.
Book
How do birds navigate so successfully over thousands of kilometres making a return trip which may take them to the same nesting site every year? By the mid 1980s leading researchers thought that this most enduring and endearing of puzzles was solved and the author, a well known figure in the navigation field, explored this view in the light of the then most recent discoveries. The book is written primarily for the serious student of biology but the manner of presentation, with a strong visual edge, puts it within reach of a wider readership of those interested in birds, navigation or migration. At first sight the available evidence is confused and contradictory; when pieced together in a simple way, however, a coherent picture emerges of how birds seem to navigate. Theory and evidence are presented hand in hand on a determined onslaught on a major biological enigma. There are clear examples of the importance of rigour in experimental design, and potential researchers will find encouragement from the example of some scientists who, despite initial ridicule from their establishment, have through scientific thoroughness and determination succeeded eventually in pioneering major advances.
HE aircraft was on final approach through the rain and fog. At ap- proximately 500 feet it went into a spiral dive to starboard, striking the right wing against the approach lights. The aircraft was destroyed. Blackburnian Warblers were migrating on a night of a low ceiling and the visibility restricted in moderate rain. On reaching a floodlighted area, some fifty birds crashed into a hangar and were killed. Initially there does not seem to be much in common, except the weather, in these two unfortunate occurrences. Yet, under analysis, there may be a great deal of similarity. In both cases the fliers were attempting to fly through deteriorating weather conditions, pickin g their way through a maze of light and shadow, of reflected and refracted light shining through rain, an opaque obstructing medium. It is suggested that the cause of both the crashes was exactly the same. The fliers became confused by the abruptness of intense lighting, and, using the primary sense of orientation (sight) in conjunction with erroneous sensory stimuli, suffered a complete loss of spatial orientation. Birds, particularly the nocturnal migrants when flying at low level are susceptible to, and suffer from vertigo and spatial disorientation the same as man. For the purpose of this discourse it is assumed that: (a) the aerodynamic forces acting on the wings of a bird are the same as those acting on the wings of an aircraft; (b) only nocturnal migrants are under consideration; (c) the sense organs are used for the same basic purposes in both birds and man; and (d) while the senses of the bird may be more acute, the psycho- physiological reactions to the stimuli are similar in birds and man. Although the aerial environment is applicable to both birds and man, each has its own peculiar environment in which it flies and this is not readily examinable by direct observation. This environment is made up of stimuli appreciated by sensory organs and perceived by the brain. The reactions to the stimuli are based upon knowledge, and each reaction must be correct in the proper place at the proper time. Because the human pilot is sensitive to similar stimuli, man can visualize the aerial world of other fliers. It is only through a comparison of the bird with man's knowledge of flying that we can deduce how a bird flies. As spatial disorientation is extremely common (90 per cent incidence) amongst all-weather pilots, the most expedient way of determining the hap- penings and causes of spatial disorientation in birds is to consider first the human pilot.