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Abstract

Iteroparous organisms face a trade-off between reproduction and survival but knowledge of whether, how and when costs of long-term increases in workload are paid is scant. We increased locomotion costs for a whole year by equipping male great tits with a backpack during breeding, removing the backpacks one year later. We applied three different treatments: control (without backpack), light (“empty” backpack, 0.1g) and heavy (“full” backpack, 0.9g, ~5% of body mass). Backpacks were administered in three cohorts and we monitored effects on mass of nestlings and the male, wing length, reproduction and survival. Added mass had a negative effect on nestling mass in both the starting year of the experiment and one year later, but not on production of fledglings or recruits. In winter and the next breeding season, males equipped with heavy backpacks had a higher (net) body mass and had shorter third primary feathers than the other two groups. Heavy backpack males were less likely to sleep in a nest box in winter. Nest boxes are optimal roosting sites and we interpret this finding as a treatment effect on success in competition over this resource. However, there was no effect of the manipulation on survival. Overall, we found no long-term fitness consequences and we discuss possible explanations and implications for the “starvation predation theory” of optimal body mass. However, we found short-term effects of carrying extra weight suggesting that behavioral studies using small devices should consider the effects of equipping small non-migratory passerines with devices such as transmitters.

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... First, heritability of telomere length could explain this pattern if individuals with short telomeres are more likely to reproduce at young ages. In wild vertebrates, telomere length is known to be somewhat heritable ( [49,89,90,91,92]; but see [68]) but also to be related to longevity [48,88] and individual quality [93]. Therefore, a progressive disappearance of individuals with short telomeres may occur among a cohort as individuals become older [94]. ...
... Therefore, a progressive disappearance of individuals with short telomeres may occur among a cohort as individuals become older [94]. Because telomere length is somewhat heritable [49,89,90,91,92], this could translate into a positive correlation between parental age and offspring telomere length and may therefore explain our results ("the selection hypothesis"). Supporting this possibility, survival probability is reduced at young ages in Black-browed albatrosses [9]. ...
... This may result from a large inter-individual variance in both telomere length and body condition that might have blurred the relationship between telomere length and body condition in our study. Offspring telomere length is certainly affected by developmental conditions, but also by initial telomere length through heritability [49,89,90,91,92]. In addition, body condition is highly labile in old albatross chicks because parent albatrosses feed their large chicks quite irregularly [78]. ...
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In wild vertebrates, young parents are less likely to successfully rear offspring relative to older ones because of lower parental skills (‘the constraint hypothesis’), lower parental investment (‘the restraint hypothesis’) or because of a progressive disappearance of lower-quality individuals at young ages (‘the selection hypothesis’). Because it is practically difficult to follow an offspring during its entire life, most studies have only focused on the ability of individuals to breed or produce young, while neglecting the ability of such young to subsequently survive and reproduce. Several proxies of individual quality can be useful to assess the ability of young to survive and recruit into the population. Among them, telomere length measurement appears especially promising because telomere length has been linked to longevity and fitness in captive and wild animals. By sampling 51 chicks reared by known-aged parents, we specifically tested whether parental age was correlated to offspring telomere length and body condition in a long-lived bird species, the Black-browed Albatross (Thalassarche melanophrys). Young Black-browed albatrosses produced chicks with shorter telomere relative to those raised by older ones. Short offspring telomeres could result from poor developmental conditions or heritability of telomere length. Moreover, young parents also had chicks of lower body condition when compared with older parents, although this effect was significant in female offspring only. Overall, our study demonstrates that parental age is correlated to two proxies of offspring fitness (body condition and telomere length), suggesting therefore that older individuals provide better parental cares to their offspring because of increased parental investment (restraint hypothesis), better foraging/parental skills (constraint hypothesis) or because only high-quality individuals reach older ages (selection hypothesis).
... Individual great tits were identified by ring number and unringed individuals were ringed. The overall capture probability of individuals known to be alive was 88% in our population (Atema et al., 2016), hence we were able to estimate survival based on catches of the adults in both winter and spring. ...
... The experiment included three treatments: (i) control without a backpack (n = 97), (ii) "light" with an empty backpack of 0.1 g as a control for possible effects of the harness (n = 68) and (iii) 'heavy' with a backpack of 0.9 g (~5% of body mass; n = 80). Further details can be found in Atema et al. (2016), ...
... where we show there were no long-term fitness consequences of the manipulation, although there were short-term effects of the backpacks on male state: males carrying backpacks slightly increased their mass, and were less successful in securing a nest box as roosting site in winter (Atema et al., 2016). ...
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Evidence that telomere length (TL) and dynamics can be interpreted as proxy for ‘life stress’ experienced by individuals stems largely from correlational studies. We tested for effects of an experimental increase of workload on telomere dynamics by equipping male great tits (Parus major) with a 0.9 gram backpack for a full year. In addition, we analysed associations between natural life‐history variation, TL and TL dynamics. Carrying 5% extra weight for a year did not significantly accelerate telomere attrition. This agrees with our earlier finding that this experiment did not affect survival or future reproduction. Apparently, great tit males were able to compensate behaviourally or physiologically for the increase in locomotion costs we imposed. We found no cross‐sectional association between reproductive success and TL, but individuals with higher reproductive success (number of recruits) lost fewer telomere base pairs in the subsequent year. We used the TRF method to measure TL, which method yields a TL distribution for each sample, and the association between reproductive success and telomere loss was more pronounced in the higher percentiles of the telomere distribution, in agreement with the higher impact of ageing on longer telomeres within individuals. Individuals with longer telomeres and less telomere shortening were more likely to survive to the next breeding season, but these patterns did not reach statistical significance. Whether successful individuals are characterized by losing fewer or more base pairs from their telomeres varies between species, and we discuss aspects of ecology and social organisation that may explain this variation.
... In songbirds, tag mass is usually advised to not exceed a maximum of 5% of the body mass (based on Brander & Cochran, 1969, further tested on bats, Aldridge & Brigham, 1988), yet this "rule" is not grounded on scientific data. The existing data vary substantially, showing no tag effects (Bell et al., 2017;Peterson et al., 2015), effects of tags irrespective of their mass (Bowlin et al., 2010;Tomotani, Bil, et al., 2019), or effects being context rather than mass specific (Atema et al., 2016;Snijders, Nieuwe-Weme, et al., 2017). Moreover, the time period over which an animal carries a tag might play a role as birds might adapt to the increase in weight. ...
... This is because acceleration and speed of all these types of flight maneuvers depend on the ratio between aerodynamic thrust force and body (+tag) weight, expressed by the actuator disk loading. Yet, given the large number of studies not finding reproduction or survival effects of radio-tagging (Atema et al., 2016;Barron et al., 2010;Bell et al., 2017;Brlík et al., 2020;Snijders, Nieuwe-Weme, et al., 2017), it would need to be deter- Previous studies showed that songbirds with radio-tags prospect novel environments widely in a biologically meaningful way (Amrhein et al., 2004;Roth et al., 2009), radio-tagged resident birds maintain their home ranges and social interactions (Bircher et al., 2020(Bircher et al., , 2021Naguib et al., 2001;Snijders et al., 2014), and migratory birds are capable of completing their migratory journeys (Bridge et al., 2013;Ouwehand et al., 2016;Peterson et al., 2015;Stutchbury et al., 2009;Tomotani, de la Hera, et al., 2019 and many others). ...
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The use of biologging and tracking devices is widespread in avian behavioral and ecological studies. Carrying these devices rarely has major behavioral or fitness effects in the wild, yet it may still impact animals in more subtle ways, such as during high power demanding escape maneuvers. Here, we tested whether or not great tits (Parus major) carrying a backpack radio-tag changed their body mass or flight behavior over time to compensate for the detrimental effect of carrying a tag. We tested 18 great tits, randomly assigned to a control (untagged) or one of two different types of a radio-tag as used in previous studies in the wild (0.9 g or 1.2 g; ~5% or ~6–7% of body mass, respectively), and determined their upward escape-flight performance 1, 7, 14, and 28 days after tagging. In between experiments, birds were housed in large free-flight aviaries. For each escape-flight, we used high-speed 3D videography to determine flight paths, escape-flight speed, wingbeat frequency, and actuator disk loading (ratio between the bird weight and aerodynamic thrust production capacity). Tagged birds flew upward with lower escape-flight speeds, caused by an increased actuator disk loading. During the 28-day period, all groups slightly increased their body mass and their in-flight wingbeat frequency. In addition, during this period, all groups of birds increased their escape-flight speed, but tagged birds did so at a lower rate than untagged birds. This suggests that birds may increase their escape-flight performance through skill learning; however, tagged birds still remained slower than controls. Our findings suggest that tagging a songbird can have a prolonged effect on the performance of rapid flight maneuvers. Given the absence of tag effects on reproduction and survival in most songbird radio-tagging studies, tagged birds in the wild might adjust their risk-taking behavior to avoid performing rapid flight maneuvers.
... It was expected that attaching a device to a bird would cause a variety of negative effects, not only in terms of survival, but also behavioural changes (Barron, Brawn, & Weatherhead, 2010;Costantini & Møller, 2013), raising both scientific and ethical concerns. For example, individuals may suffer from lowered reproductive output and lowered return rates (e.g., Arlt, Low, & Pärt, 2013); however, not all studies found a significant tag impact on breeding or survival, with many reporting low or no impact (e.g., Atema, Noordwijk, Boonekamp, & Verhulst, 2016;Peterson et al., 2015). ...
... In order to prevent any variation in tag manufacturing, we constructed a single "light tag," a single "medium tag", and a single "heavy tag," each with dimensions of 13 × 8 × 8 mm (length × width × height). Birds may undergo physiological changes after tag attachment (Atema et al., 2016); to prevent this, we minimized tag exposure by fitting the tags to the birds immediately before each test and removed them as soon as the test was finished. To do that, 2 days before the second "flight day" (the first experimental "flight day"), we fitted a leg-loop harness with a piece of Velcro to the bird, which allowed us to easily attach the dummy tag before each "flight day" and remove it afterwards with minimal handling time and stress for the bird (Figure 1b). ...
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1.The recent boost in bird migration studies following the development of various tracking devices raised awareness of how detrimental attaching devices can be for animals. Such effects can occur during migration, but also immediately post‐release if the device impairs escape flight performance and, consequently, the bird's ability to evade predators. 2.In this study, we investigated the effect of carrying a device on the escape flight speed and aerodynamic force production in a migratory passerine. We recorded upward‐directed escape flights of 15 male blackcaps. Each individual was tested without a tag, and when equipped with three different leg‐loop dummy tags with masses representing around 3%, 5% and 7% of their body mass. The experiment was designed such that all individuals passed through all treatments in a randomized order. 3.We found that two factors affected flight speed in roughly equal amounts: first, tagged escape flights had lower flight speeds compared to the control flights, irrespective of tag mass. Second, we found an effect of the total mass, i.e. the sum of the masses of the individual bird and of the tag, with heavier birds being slower. In contrast, flight speed was not correlated with relative tag mass in percentage of body mass, the metric commonly used in ethical guidelines for tag attachment. Aerodynamic flight force production also depended on total mass, with heavier birds producing higher forces. But these flight forces did not differ between flights with or without a tag. 4.We conclude that, when tagging birds, it is misleading to choose heavy individuals for tagging in order to minimize the tag mass as a percentage of body mass. This is particularly relevant in species for which body mass is not necessarily related to size, like migratory birds that accumulate large fat reserves. The lower escape speed in “tagged” flights could not be explained by differences in net flight force production, because these did not differ between flights with and without a tag. This suggests that the tag also affected pre‐flight take‐off dynamics, possibly due to a leg harness‐induced reduction in leg push‐off performance. This article is protected by copyright. All rights reserved.
... Great tits are an ideal species for quantifying such tag effects since the reproduction and provisioning behaviour of comparable untagged birds can also be accurately monitored, facilitating identification of tag induced effects. For example, a recent study showed no negative long-term effects for male great tits fitted with ca. 5% backpacks during nestling provisioning (Atema et al., 2016). ...
... Yet, two previous great tit tracking studies did not find such negative effects of tagging during nestling provisioning (chicks between 8 and 17 days of age), using 4%-5% tags (van Overveld et al., 2011;Atema et al., 2016), so we cannot rule out that the use of heavier tags in our study made an important difference. However, both these studies only tagged one of the parents, and this reduced disturbance might have been as important as the lower tag weights in preventing nest desertion. ...
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Biotelemetry devices provide unprecedented insights into the spatial behaviour and ecology of many animals. Quantifying the potential effects of attaching such devices to animals is essential, but certain effects may appear only in specific or particularly stressful contexts. Here we analyse the effects of radio tag attachment on great tits (Parus major) tagged over three environmentally dissimilar years, as part of a project studying social- and communication networks. When we radio-tagged birds before breeding, only those tagged in the coldest spring tended to be less likely to breed than control birds. Breeding probability was independent of the relative tag weight (between 5% and 8% bodyweight). When we radio-tagged both parents during nestling provisioning (tag weight between 6% and 8%), tagged parents were more likely than control parents to desert their brood in two of the three years, while in the other year no tagged parents deserted. Tagged parents provisioning larger broods were most likely to desert, especially during lower average temperatures. Video analyses did not reveal any tag effects on provisioning behaviour of parents in the year with no desertion. We conclude that radio tagging before breeding did not lead to negative effects, regardless of tag weight, but that decisions about radio-tagging parents during nestling provisioning need to be made with exceptional care, taking both environmental context and tag weight into account. Reporting results from long-term radio-tracking studies comprising several environmentally variable years is crucial to understand and predict potential tag effects and maximise the tremendous potential of biotelemetry. This article is protected by copyright. All rights reserved.
... To assess the differences among the posterior distributions of our brood size manipulation treatments we used the Kullback-Leibler divergence calibration (KLDC) (Kullback & Leibler 1951;McCulloch 1989;Karabatsos 2006;Boonekamp et al. 2014;Atema et al. 2016;Rodr ıguez-Muñoz et al. 2019), which show the mean Kullback-Leibler discrepancies (KL) between two distributions. Values closer to 0.5 imply that there is minimal difference among the distributions and values closer to 1 imply major differences (Kullback & Leibler 1951;McCulloch 1989). ...
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Early‐life conditions can have long‐lasting effects and organisms that experience a poor start in life are often expected to age at a faster rate. Alternatively, individuals raised in high‐quality environments can overinvest in early‐reproduction resulting in rapid ageing. Here we use a long‐term experimental manipulation of early‐life conditions in a natural population of collared flycatchers (Ficedula albicollis), to show that females raised in a low‐competition environment (artificially reduced broods) have higher early‐life reproduction but lower late‐life reproduction than females raised in high‐competition environment (artificially increased broods). Reproductive success of high‐competition females peaked in late‐life, when low‐competition females were already in steep reproductive decline and suffered from a higher mortality rate. Our results demonstrate that ‘silver‐spoon’ natal conditions increase female early‐life performance at the cost of faster reproductive ageing and increased late‐life mortality. These findings demonstrate experimentally that natal environment shapes individual variation in reproductive and actuarial ageing in nature.
... The effectiveness of a group's antipredator response will depend upon its cohesion and the effective participation of all of its members (i.e. the greater the number of individuals on high alert, the greater will be the distance at which danger is detected). The size of a bird therefore affects its social relationships, physiology and ecology in a variety of ways 22 . ...
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Wild birds differ in size according to their age and sex, adult birds being larger than juveniles. In the galliforms, males are larger than females, in contrast to some groups, such as the raptors, in which the females are larger. Size generally influences the rank hierarchy within a group of birds, although the age, sex, temperament and behaviour of an individual may override its size related rank order. The scaled size of birds according to age and sex affects their physiology and behaviour. Precise details of body-size differences by age and sex are poorly known in most partridge species. We measured 13,814 wild partridges in a homogenous population over 14 years of study to evaluate size differences within a uniform habitat and population management regime. We show that wild Red-legged Partridges have scaled mass, and body- and wing-lengths consistent with age/sex classes. Power functions between mass and body-length (as a proxy for walking efficiency), and between mass and wing-length (for flight efficiency) differ between juvenile females and males, and adult females and males. We discuss these findings and their physiological, behavioural and ecological implications.
... While studies of tag effects have mainly focused on apparent survival probabilities (Barron et al. 2010), there has been limited study of potential 'carry-over' effects of additional tag load on arrival dates to breeding grounds, timing of breeding or investment in clutch size (Arlt et al. 2013(Arlt et al. , G omez et al. 2014. Studies examining the impacts of tags on dependent young are particularly limited (although see Rodriguez et al. 2009, Atema et al. 2016, Rodr ıguez-Ruiz et al. 2016, Weiser et al. 2016, with only two such studies on migratory passerine birds (Arlt et al. 2013, Scandolara et al. 2014). This paucity partly results from passerine tracking studies being conducted on adult birds that are not associated with a monitored nest. ...
Article
Tracking small passerines using miniaturised location tags is a rapidly expanding field of study. In a one-year study, we tested whether there were any short- or longer-term effects of fitting geolocators weighing 3% of body mass on male Pied Flycatchers Ficedula hypoleuca. In the deployment year, we compared adult provisioning rates to nestlings, nestling growth and nest success between nesting attempts in which adult males were fitted with a geolocator, with control nests where males had the same capture history but were not tagged. We found no difference between treatments in provisioning effort by males or their associated female two days after geolocator fitting, in terms of nestling growth, subsequent brood reduction or nest success. Return rate, arrival date on territories, timing and breeding parameters were compared between tagged and untagged males in the following breeding season. We found no difference in return rate or arrival date, and no difference in nest timing, fecundity or outcome. Our study suggests that fitting lightweight tags to small passerines need not affect behaviour, breeding or apparent between-year survival. However, tagging new species should still require assessment and comparison with well-matched control cohorts, and it should be recognised that tag effects could vary between years and populations, mediated by environmental conditions. This article is protected by copyright. All rights reserved.
... One important resource for which competition in winter may occur is the availability of roosting boxes. Roosting in a nest box, as opposed to roosting outside, may enhance winter survival of birds by decreasing thermoregulatory costs and the risk of predation (Atema, Van Noordwijk, Boonekamp, & Verhulst, 2016;Drent, 1987;Mainwaring, 2011). We expected that if the availability of roosting boxes was limited, experimentally manipulated family size would negatively affect the ability of parents to claim a roosting box in the subsequent winter. ...
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Reproductive behavior cannot be understood without taking the local level of competition into account. Experimental work in great tits (Parus major) showed that (1) a survival cost of reproduction was paid in environments with high levels of competition during the winter period and (2) experimentally manipulated family size negatively affected the ability of parents to compete for preferred breeding boxes in the next spring. The fact that survival was affected in winter suggests that the competitive ability of parents in winter may also be affected by previous reproductive effort. In this study, we aim to investigate whether (1) such carryover effects of family size on the ability of parents to compete for resources in the winter period occurred and (2) this could explain the occurrence of a survival cost of reproduction under increased competition. During two study years, we manipulated the size of in total 168 great tit broods. Next, in winter, we induced competition among the parents by drastically reducing the availability of roosting boxes in their local environment for one week. Contrary to our expectation, we found no negative effect of family size manipulation on the probability of parents to obtain a roosting box. In line with previous work, we did find that a survival cost of reproduction was paid only in plots in which competition for roosting boxes was shortly increased. Our findings thus add to the scarce experimental evidence that survival cost of reproduction are paid under higher levels of local competition but this could not be linked to a reduced competitive ability of parents in winter. K E Y W O R D S brood size manipulation, density dependence, frequency dependence, parental care, social environment
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Telomere length (TL) is increasingly used as a biomarker of senescence, but measuring telomeres remains a challenge. Within tissue samples, TL varies between cells and chromosomes. Class I telomeres are (presumably static) interstitial telomeric sequences, and terminal telomeres have been divided in shorter (Class II) telomeres and ultra‐long (Class III) telomeres, and the presence of the latter varies strongly between species. Class II telomeres typically shorten with age, but little is known of Class III telomere dynamics. Using multiple experimental approaches, we show great tits to have ultra‐long telomeres, and we investigated age effects on Class II and III telomeres using a longitudinal approach (our method excludes Class I telomeres). In adults, TL averaged over the whole distribution did not significantly change with age. However, more detailed analyses showed that Class II TL did shorten with age, and, as in other species, the longest Class II telomeres within individuals shortened faster with age. In contrast, Class III TL did not shorten with age within individual adults. Surprisingly, we found the opposite pattern in nestlings: Class III TL shortened significantly with age, while the age effect on Class II TL was close to zero. Thus, Class III telomere length may provide information on developmental history, while Class II telomere length provides information on telomere dynamics in adulthood. These findings have practical implications for telomere studies and raise the interesting question what causes variation in TL dynamics between chromosomes within individuals and how this is related to development. This article is protected by copyright. All rights reserved.
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Light-level geolocators, miniature devices used for tracking avian migration over the full annual cycle, are being widely deployed on small migratory passerines. However, the effects of carrying geolocators on the breeding biology of songbirds are unclear, and variable species- and guild-specific conclusions have been drawn regarding their effects on return rates (apparent annual survival). In particular, there is a lack of published information on the effects of geolocators on Nearctic-Neotropical migrant warblers and canopy-dwelling bird species, which limits our ability to determine whether this technology is appropriate for use on species within these groups. During 2014 and 2015, we deployed geolocators on 49 adult male Cerulean Warblers (Setophaga cerulea) in Pennsylvania, Missouri, and Arkansas, USA. We monitored the effects of geolocators across the full annual cycle by comparing apparent within-breeding-season survival (within-season φ), nestling provisioning rates, nest survival, and return rates between geolocator-tagged adult males and color-banded controls. We found no negative effects of geolocators during the breeding season of geolocator deployment, but the return rate of geolocator-tagged birds was lower than that of control birds (16% ± 5% vs. 35% ± 7%). We found no strong evidence that the differential return rate between the 2 groups was influenced by breeding region, body mass, bird age, year of geolocator deployment, or method of attachment. Although finding no effect of geolocators during the breeding season is encouraging, the lower return rate of geolocator-tagged birds warrants further investigation in the field. If further improvements in the design or attachment methods of geolocators are not technologically possible, the potential for increased mortality (or dispersal) of geolocator-tagged birds should be weighed against the potential conservation gains that could be made by identification of critical stopover, wintering, and breeding habitats for populations of interest.
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Life-history theory predicts a trade-off for allocation of limited resources to reproduction and self-maintenance; however, many of the underlying physiological mechanisms remain elusive. There is growing evidence for oxidative stress to play an essential role in this trade-off because some by-products from the immune system and from normal metabolism generate reactive oxygen species that can cause oxidative damage. We manipulated reproductive effort of male and female great tits shortly before reproduction by clipping feathers of either the male or female parent of pairs of known age, given that parental effort may differ between the sexes and change over the lifetime of an individual. We quantified the effect of the treatment on morphological, physiological, behavioral, and reproductive traits. We found that feather clipping led to a decrease in parental body mass and to a reduced clutch size. Nestlings raised by clipped fathers showed reduced body mass although feeding rate was equally high between clipped and control individuals. In contrast to our predictions, we found that the feather clipping did not affect oxidative status. However, independently of the treatment, adult males had higher antioxidant capacity than females and older males showed higher oxidative damage compared with yearlings. Thus, our results suggest that the self-maintenance was prioritized over reproduction. It suggests that males are more susceptible to increased workload than females and thus more likely to reduce allocation of resources to reproduction.
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The advantages and disadvantages of breeding in nestboxes are well known, whilst the merits of roosting in nestboxes during the non-breeding season remain poorly understood. Here I review the advantages and disadvantages of using nestboxes as roosting sites during the non-breeding season. The main advantage of nestboxes is that they increase the number of cavities available for roosting and birds gain considerable thermal benefits and energy savings, when compared to congeners roosting outside of cavities. However, the main disadvantage is that roosting birds are widely targeted by detrimental ectoparasites, and birds actively avoid nestboxes where ectoparasites are abundant. Meanwhile, there is insufficient evidence to make any firm conclusions as to whether roosting in nestboxes increases or decreases predation risk and this is an area, which warrants further research. Consequently, there is a great deal of variation in the quality of individual nestboxes as roosting sites, and interspecific competition results in larger and more dominant species roosting in preferred nestboxes. In summary, this review emphasises the importance of nestboxes to roosting birds during the non-breeding season.
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With a newly developed small transmitter, radiotracking of birds with a minimum body mass of 7 g is possible. Its high pulse rate (4-9 Hz) allows birds to be located within a few seconds with a precision of ±lAO (SD), i.e. ±2A m at 100 m bearing distance. Data on the use of indi-vidual trees by Great and Blue Tits (Parus major and P. caeruleus) revealed details of home-range use and separation of the home-ranges of neighbour-ing birds. Within 1 min, Great and Blue Tits moved over a median distance of 18 m and 10m, respectively. The range use of the tits was determined to a great extent by the spatial distribution of caterpillars, the main prey dur-ing breeding season. Overlaps of neighbouring home-ranges were observed only for areas of low location density. By contrast, the central parts of the home-ranges were used exclusively by the resident pairs. Using the exam-ples given, I discuss four main methodical aspects of radiotracking of small woodland birds: (1) Location probability and basic qualities of the data ob-tained. (2) Spatial and temporal resolution of tracking data. (3) Tracking of short and medium distance movements. (4) Analysis of territoriality and re-lationships between neighbours. The new technique greatly improves the observation of small animals under poor visibility conditions. It is, howev-er, limited to a relatively small number of individuals, which has its impli-cations for study design and statistical analysis of the data.
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Behaviour may contribute to changes in fitness prospects with age, for example through effects of age dependent social dominance on resource access. Older individuals often have higher dominance rank, which may reflect a longer lifespan of dominants, and / or an increase in social dominance with age. In the latter case, increasing dominance could mitigate physiological senescence. We studied the social careers of free-living jackdaws over a twelve-year period, and found that (i) larger males attained higher ranks, (ii) social rank increased with age within individuals, and (iii) high-ranked individuals had shorter lifespan suggesting that maintaining or achieving high rank and associated benefits comes at a cost. Lastly, (iv) social rank declined substantially in the last year an individual was observed in the colony, and through its effect on resource access this may accelerate senescence. We suggest that behaviour affecting the ability to secure resources is integral to the senescence process via resource effects on somatic state, where behaviour may include social dominance but also learning, memory, perception and (sexual) signalling. Studying behavioural effects on senescence via somatic state may be most effective in the wild, where there is competition for resources, which is usually avoided in laboratory conditions.
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Estimates of selection in natural populations are frequent but our understanding of ecological causes of selection, and causes of variation in the direction, strength and form of selection, is limited. Here, we apply a multi-level framework to partition effects of great tit fledging mass on first-year survival to hierarchical levels and quantify their ecological dependence using a data set spanning 51 years. We show that estimates of the effect of fledging mass on first-year survival decline three-fold from year- to brood- to individual-level, so that estimates of selection depend strongly on the level at which they are calculated. We identify variables related to summer and winter food availability as underlying higher-level effects of fledging mass on first-year survival, and show experimentally that brood-level effects originate early in development. Further, we show that predation and conspecific density modulate individual-level effects of fledging mass on first-year survival. These analyses demonstrate how correlations between traits, fitness and environment influence estimates of selection, and show how partitioning trait-effects between levels of selection and environmental factors is a promising approach to identify potential agents of selection. This article is protected by copyright. All rights reserved.
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Maximum likelihood or restricted maximum likelihood (REML) estimates of the parameters in linear mixed-effects models can be determined using the lmer function in the lme4 package for R. As for most model-fitting functions in R, the model is described in an lmer call by a formula, in this case including both fixed- and random-effects terms. The formula and data together determine a numerical representation of the model from which the profiled deviance or the profiled REML criterion can be evaluated as a function of some of the model parameters. The appropriate criterion is optimized, using one of the constrained optimization functions in R, to provide the parameter estimates. We describe the structure of the model, the steps in evaluating the profiled deviance or REML criterion, and the structure of classes or types that represents such a model. Sufficient detail is included to allow specialization of these structures by users who wish to write functions to fit specialized linear mixed models, such as models incorporating pedigrees or smoothing splines, that are not easily expressible in the formula language used by lmer.
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Optimality theories of ageing predict that the balance between reproductive effort and somatic maintenance determines the rate of ageing. Laboratory studies find that increased reproductive effort shortens lifespan, but through increased short-term mortality rather than ageing. In contrast, high fecundity in early life is associated with accelerated senescence in free-living vertebrates, but these studies are non-experimental. We performed lifelong brood size manipulation in free-living jackdaws. Actuarial senescence--the increase in mortality rate with age--was threefold higher in birds rearing enlarged- compared to reduced broods, confirming a key prediction of the optimality theory of ageing. Our findings contrast with the results of single-year brood size manipulation studies carried out in many species, in which there was no overall discernible manipulation effect on mortality. We suggest that our and previous findings are in agreement with predictions based on the reliability theory of ageing and propose further tests of this proposition.
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Researchers often attach radio transmitters and other devices to free-living birds without a clear understanding of the possible consequences for their study organisms or their data. Although transmitters may affect parental investment (nest defense and offspring provisioning), this possibility has received little attention. We tested this hypothesis by placing mock radio transmitters on male Northern Cardinals (Cardinalis cardinalis) and comparing their behavior to that of uncaptured birds and procedural controls. Birds with transmitters defended their nests less vigorously than did uncaptured birds but did not modify their provisioning effort. This behavioral modification appears to have ultimately influenced predation rates, as nests of birds with transmitters had lower daily survival rates and were less likely to fledge offspring. Control birds that were captured, handled, and bled had intermediate levels of nest defense and productivity that were statistically indistinguishable from those of birds receiving other treatments, suggesting that capture, restraint, and blood collection may affect birds in ways that are independent of transmitters' effects. Interestingly, we also found limited evidence that females mated to males with transmitters increased their provisioning effort, possibly in compensation for a perceived reduction in their mate's care. Because attachment of a transmitter (and potentially blood sampling) directly affected the behavior and reproduction of birds with transmitters and may have indirectly affected the behavior of their mates, we suggest researchers cautiously balance the benefits of such methods against potential data biases and impairment of reproduction.
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Energy expenditure in wild animals can be limited (i) intrinsically by physiological processes that constrain an animal's capacity to use energy, (ii) extrinsically by energy availability in the environment and/or (iii) strategically based on trade-offs between elevated metabolism and survival. Although these factors apply to all individuals within a population, some individuals expend more or less energy than other individuals. To examine the role of an energy ceiling in a species with a high and individually repeatable metabolic rate, we compared energy expenditure of thick-billed murres (Uria lomvia) with and without handicaps during a period of peak energy demand (chick-rearing, N = 16). We also compared energy expenditure of unencumbered birds (N = 260) across 8 years exhibiting contrasting environmental conditions and correlated energy expenditure with fitness (reproductive success and survival). Murres experienced an energy ceiling mediated through behavioural adjustments. Handicapped birds decreased time spent flying/diving and chick-provisioning rates such that overall daily energy expenditure remained unchanged across the two treatments. The energy ceiling did not reflect energy availability or trade-offs with fitness, as energy expenditure was similar across contrasting foraging conditions and was not associated with reduced survival or increased reproductive success. We found partial support for the trade-off hypothesis as older murres, where prospects for future reproduction would be relatively limited, did overcome an energy ceiling to invest more in offspring following handicapping by reducing their own energy reserves. The ceiling therefore appeared to operate at the level of intake (i.e. digestion) rather than expenditure (i.e. thermal constraint, oxidative stress). A meta-analysis comparing responses of breeding animals to handicapping suggests that our results are typical: animals either reduced investment in themselves or in their offspring to remain below an energy ceiling. Across species, whether a handicapped individual invested in its own energy stores or its offspring's growth was not explained by life history (future vs. current reproductive potential). Many breeding animals apparently experience an intrinsic energy ceiling, and increased energy costs lead to a decline in self-maintenance and/or offspring provisioning.
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1. Costs of reproduction have been assessed experimentally by measuring subsequent survival and reproduction of parent animals raising enlarged and reduced numbers of offspring. Reported effects on survival have so far always referred to local survival of marked individuals in the study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduced survival or to an increased tendency to emigrate from the study area. Therefore, it is important to assess mortality rates in connection with brood size experiments 2. We report an analysis of the time of death in 63 cases where kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60% of the parents raising two extra nestlings were reported dead before the end of the first winter, compared to 29% of those raising control or reduced broods. This result confirms our interpretation of the manipulation effects on local survival as due to mortality rather than emigration. The extra mortality occurred in the winter following the brood enlargement 3. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. Increased parental effort in this species thus entails an increased risk of death half a year later.
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Animal-borne logging or telemetry devices are widely used for the measurements of physiological and movement data from free-living animals. For such measurements to be relevant, however, it is essential that the devices themselves do not affect the data of interest. A recent meta-analysis (Barron et al. 2010; Methods Ecol Evol. 1:180-187) reported an overall negative effect of these devices on the birds that bear them, i.e. on nesting productivity, clutch size, nest initiation date, offspring quality, body condition, flying ability, foraging behaviours, energy expenditure or survival rate. Method of attachment (Harness, Collar, Glue, Anchor, Implant, Breast-mounted, Tailmount) had no influence on the strength of these effects but anchored and implanted transmitters had the highest reported rates of device-induced mortality. Furthermore, external devices, but not internal devices, caused an increase in 'device-induced behaviour' (comfort behaviours such as preening, fluffing and stretching, and unrest activities including unquantifiable 'active' behaviours). These findings suggest that, with the exception of device-induced behaviour, external attachment is preferable to implantation. In the present study we undertake a meta-analysis of 183 estimates of device impact from 39 studies of 36 species of bird designed to explicitly compare the effects of externally-attached and surgically-implanted devices on a range of traits, including condition, energy expenditure, and reproduction. In contrast to Barron et al., we demonstrate that externally-attached devices have a consistent detrimental effect (i.e., negative influences on body condition, reproduction, metabolism, and survival), whereas implanted devices have no consistent effect. We also show that the magnitude of the negative effect of externally attached devices decreases with time. We therefore conclude that device implantation is preferable to external attachment, providing that the risk of mortality associated with the anaesthesia and surgery required for implantation can be mitigated. We recommend that studies employing external devices use devices that can be borne for long periods, and, wherever possible, deploy devices in advance of the time period of interest.
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Theoretical studies have suggested that birds in winter should carry higher energy reserves when food supply is lower, or less predictable, in order to maximize their probability of survival until the end of winter. In the Willow Tit Parus montanus, a passerine wintering in dominance-structured flocks, subdominant birds were found to carry higher energy reserves than dominant birds (Ekman and Lilliendahl 1993, Behav. Ecol. 4; 232-238). Since food supply is probably lower (and less predictable), for subdominant birds, this seemed in agreement with theoretical results. However, we analysed the effect of social dominance on energy reserves using data from another Willow Tit study (Hogstad 1987, Auk 104: 333-336), and found that in this study dominant birds carried the highest energy reserves. In Willow Tits, social dominance is known to affect predation risk during foraging. Using a simple analytical model, we show that when social dominance affects predation risk while foraging, but not food acquisition rate, the optimal level of energy reserves is higher for dominant than for subdominant birds. When social dominance affects both food acquisition rate and predation risk, its effect on the optimal level of energy reserves depends on the relative importance of these two factors. Thus variation in the effect of social dominance on food acquisition rate and predation risk may explain the difference between the two studies compared.
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1. Researchers often attach transmitters and other devices to free-living birds without a clear understanding of potential deleterious consequences to their study organisms, and thus to their data. Studies investigating this topic have generally been limited to a single species or type of device. 2. To achieve a broader understanding we used a meta-analysis of 84 studies to ask: (1) Do devices have an overall effect on birds? (2) Which aspects of avian behaviour and ecology are affected? (3) What attributes of birds influence transmitter effects? (4) What attributes of devices influence their effects? (5) Are effects partially a consequence of capture and restraint? 3. We found a significant negative effect of devices on birds, both overall and for 8 of the 12 specific aspects analysed. The most substantial effects were that birds with devices had markedly increased energy expenditure and were much less likely to nest. 4. Effects were independent of attributes of the birds (sex, age, primary method of locomotion and body mass). We also found no evidence that proportionally heavier devices had greater effects, although researchers generally avoided using heavy devices. Breast-mounted and harness attachments increased device-induced behaviours such as preening, however, and the risk of device-induced mortality differed between attachment methods. 5. Other than foraging behaviours, no effects were a consequence of capture or restraint. 6.Synthesis and applications. We provide the first comprehensive evidence that transmitters and other devices negatively affect birds and may bias resulting data. Researchers should balance the benefits of using these techniques against potential costs to the birds and reliability of the data obtained.
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Parental care has been widely studied in birds and mammals, but variation in space use in family groups has received less attention, despite its potential importance for both survival and subsequent dispersal of offspring. In this study, we evaluate factors affecting postfledging family space use in a small territorial songbird, the great tit (Parus major). Family space use was monitored using radio tracking. Our main objectives were 1) to quantify in detail the temporal and spatial scale of family movements, 2) to test behavioral hypotheses explaining when and how frequently families leave their breeding territory, and 3) to test to what extent movements were based on familiarity with the environment. We found that variation in space use was to a large extent due to some families, but not others, regularly undertaking foraging excursions of up to more than a kilometer away. Daily excursion probability was higher for families occupying low-quality territories, and consequently, these families covered larger areas during foraging. Excursion behavior and range use also strongly depended on maternal breeding experience and personality. We further present some striking examples of inexperienced mothers moving toward previously visited areas, suggesting that familiarity with the environment plays an important role in patterns of space use. Overall, our results suggest that variation in family movements reflects different foraging strategies in relation to parental characteristics. Copyright 2011, Oxford University Press.
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Individuals differ in their ability to cope with energetically demanding situations while caring for the current brood, and they can signal this ability by their colouration. We examined the impact of handicapping (clipping of wing and tail feathers) on an energetically demanding care behaviour (incubation) in female Great Tits (Parus major). We hypothesised that the intensity of carotenoid-based breast feather colouration signals the ability to cope with impaired flight ability and the consequent increased energetic demands. If this is the case, females with more intensely coloured feathers should cope better with the handicap compared with less intensely coloured females, i.e. the impact of handicapping on mass loss and nest attentiveness should be negatively correlated with colouration. Handicapped females lost more weight than control females but did not decrease nest attentiveness to a greater extent, suggesting that females take the costs of handicapping on themselves. Females in poor condition were more severely influenced by handicapping. Intensity of female breast feather colouration did not correlate with either change in nest attentiveness or body mass loss during incubation. Intensity of breast feather colouration therefore does not appear to signal female ability to cope with this energetically demanding situation during incubation.
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1. Quantification of the energetic needs of reproducing animals provides a basis for understanding patterns in reproduction. The doubly labelled water technique enables this to be carried out under natural conditions. 2. Daily energy expenditure of 32 female Great Tits (Parus major) tending nestlings 11-12 days old (DEE(par)) and energy expenditure at night of five females was measured using the doubly labelled water technique. 3. Average DEE(par) was 95.1 kJ (24 h)-1 (1.10 W) and close to most predictions based on interspecific allometric relationships. Night metabolism was estimated to be 0.68 W. 4. Individual variation in DEE(par) could be explained by variation in body mass (+), ambient temperature (-), clutch size (+) and food availability (-) but not by female tarsus length, age, brood size or nestling mass. The significant factors together accounted for the differences found between years and broods and explained 64% of the variation. Possible causal pathways are discussed. 5. The D
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The process of ageing was long thought to be too infrequent to affect life-histories in natural populations. Long-term studies have, however, recently demonstrated ageing to be ubiquitous even in the wild, although confounding factors, such as emigration instead of mortality, or inter-population variation in rates of ageing have seldom been addressed. Here, we present analyses of female age-specific reproductive performance in a Dutch island population of great tits Parus major. For this population with limited connectivity to surrounding areas, we show that, between individuals, reproductive lifespan positively co-varies with recruit production, while within individuals performance improves up to 3 years of age, after which it gradually declines. We also show these patterns to be strikingly similar to those recently found in a less isolated British mainland population of great tits, characterised by different environmental conditions and life-history strategies, in particular the frequency of multiple breeding. Our results therefore suggest patterns of age-specific reproductive performance to be robust to both environmental and life-history variation.
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We measured the selection pressure on brood size in a recently established population of great tits (Parus major L.) in the northern Netherlands by manipulating brood size in three years (1995: n = 51, 1997: n = 66, 1998: n = 51), and we estimated fitness consequences in terms of local survival of both offspring and parents. Enlarged broods had highest fitness; the offspring fitness component was positively affected by manipulation and the parental fitness component was unaffected. Parental survival and the probability that parents produced a second clutch were not affected by the treatment. However, parents that had raised enlarged broods produced their second clutch later in the season. Clutch size, brood size, and laying date of birds recaptured in the next breeding season were largely independent of the treatment. We conclude that there is strong evidence for selection for larger brood size and reject the individual optimization hypothesis for this population because the number of young in the nest predicts fitness independently of the manipulation history. Copyright 2004.
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It is generally recognized that immigration and gene flow cannot be equated, but few detailed quantitative comparisons of these processes have been made over the entire lifetime of individual animals. We analyzed data collected in a longterm study of an island population of great tits Parus major, and tested two assumptions frequently made in population genetic studies. (1) The assumption that there is no difference in reproductive output between immigrant and resident breeding birds was refuted for females but not for males. Lifetime production of local recruits (LRS) was reduced by 37% in immigrant females, because female immigrants tended to leave the island after breeding, and thus reproduced for fewer years. Female LRS was density dependent, but the effect of density was independent of status (resident/immigrant). (2) The assumption that mating was random with respect to status was also refuted: assortative mating was found, even when temporal and spatial aggregation of immigrants was controlled for. Thus both assumptions were refuted, and gene flow was lower than immigration rate.
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1. Multiple breeding (raising more than one batch of young per breeding season) is a common life-history tactic, but little is known as yet of the accompanying costs and benefits. Second clutches of Great Tits, a facultative multiple breeder, were removed over three years to investigate the costs of rearing a second clutch. 2. Female parents, but not males, survived significantly better than unmanipulated control birds when the second clutch was removed. The difference between the sexes indicates a potential sexual conflict over the decision to start a second clutch. In neither sex were effects found on egg production in the next year. However, birds of both sexes produced more fledglings from first clutches in the following year when the second clutch had been removed. 3. In males, the experimental effect on fledgling production in the next year could be attributed to their mates, because this effect was restricted to males that bred with the same female as in the previous year. In females, this effect was also found among birds that bred with a new mate, which suggests that rearing a second clutch had a long-term effect. Females, but not males, found roosting in the following winter had lower mass when the second clutch had been removed, which supports the conclusion that rearing a second clutch has long-term effects. 4. The experimental effect on female survival was found in two winters with low food availability, but not in a winter with high food availability. This is in agreement with the results of a non-experimental analysis of data collected in the same population. The effect on fledgling production in the next year was also restricted to years with low food availability in the intervening winter. This suggests that the costs of rearing a second clutch depend on food availability in winter. The possibility that the costs of reproduction generally depend on environmental quality and possible mechanisms are discussed. 5. Food availability in winter is probably unpredictable at the time second clutches are started, and therefore Great Tits should ‘bet-hedge’ with regard to the decision to start a second clutch.
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1. To elucidate the links between avian brood size, parental effort and parental investment, we measured daily energy expenditure (DEEfem), condition (residuals of mass on tarsus) and feeding rate in female great tits Parus major L. rearing broods in which the number of young was either reduced, unmanipulated or enlarged. 2. Female condition was negatively correlated with manipulation when measured at the nestling age of 8 days (measured during the day), which suggests a shift in allocation from self-feeding to chick-feeding. However, there was no detectable manipulation effect on condition measured at the nestling age of 12 days (measured during the night). Either female condition was only affected by manipulation in the early nestling phase or the females adjusted their diurnal mass trajectory in response to brood size manipulation. More detailed data are required to verify this point. There were no indications of a fitness cost associated with the condition during the day, but condition at night was positively related to winter survival. Since manipulation only affected condition during the day, there was no link between manipulation and winter survival. 3. The duration of the working day was not affected by manipulation and female feeding rate tended to flatten off with manipulated brood size. Similarly, brood reduction resulted in a lower DEEfem, whilst brood enlargement had no effect. This suggests that females worked at an energetic ceiling when rearing an unmanipulated brood. However, the level of this ‘ceiling’ in DEEfem was not fixed: it differed between years. This leads us to conclude that the observed ceiling was imposed by extrinsic factors (e.g. available foraging time) and not by an intrinsic factor such as maximum energy assimilation rate. We hypothesize that time limitation was the cause for the observed ceiling in energy expenditure and that the annual variation in the level of this ceiling was due to annual variation in ambient temperature. 4. A cost of reproduction was previously demonstrated in this population: brood enlargement caused a reduction in the incidence of second clutches. However, since DEEfem did not differ between control and enlarged broods, we judge it unlikely that daily energy expenditure is a general predictor for parental investment.
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Costs of reproduction are fundamental trade-offs shaping the evolution of life histories. There has been much interest, discussion and controversy about the nature and type of reproductive costs. The manipulation of reproductive effort (e.g. brood size manipulation) may alter not only life-history traits such as future adult survival rate and future reproductive effort, but also behavioural decisions affecting recapture/resighting and dispersal probabilities. We argue that many previous studies of the costs of reproduction may have erroneously concluded the existence or non-existence of such costs because of their use of local return rates to assess survival. In this paper, we take advantage of the modern multistate capture-recapture methods to highlight how the accurate assessment of the costs of reproduction requires incorporating not only recapture probability, but also behavioural 'state' variables, for example dispersal status and current reproductive investment. The inclusion of state-dependent decisions can radically alter the conclusions drawn regarding the costs of reproduction on future survival or reproductive investment. We illustrate this point by re-analysing data collected to address the question of the costs of reproduction in the collared flycatcher and the great tit. We discuss in some detail the methodological issues and implications of the analytical techniques.
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Understanding the capacity of natural populations to adapt to their local environment is a central topic in evolutionary biology. Phenotypic differences between populations may have a genetic basis, but showing that they reflect different adaptive optima requires the quantification of both gene flow and selection. Good empirical data are rare. Using data on a spatially structured island population of great tits (Parus major), we show here that a persistent difference in mean clutch size between two subpopulations only a few kilometres apart has a major genetic component. We also show that immigrants from outside the island carry genes for large clutches. But gene flow into one subpopulation is low, as a result of a low immigration rate together with strong selection against immigrant genes. This has allowed for adaptation to the island environment and the maintenance of small clutches. In the other area, however, higher gene flow prevents local adaptation and maintains larger clutches. We show that the observed small-scale genetic difference in clutch size is not due to divergent selection on the island, but to different levels of gene flow from outside the island. Our findings illustrate the large effect of immigration on the evolution of local adaptations and on genetic population structure.
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It has been recently proposed that the blue-green coloration in eggs of many avian species may constitute a sexually selected female signal. Blue-green color intensity would reflect the physiological condition of females, and hence it might also affect the allocation of male parental care. In this study, we use three different experimental approaches to explore the importance of sexual selection on blue-green egg coloration of spotless starling (Sturnus unicolor) eggs. First, experimental deterioration of female body condition (by means of wing feather removal) negatively affected the intensity of blue-green egg coloration. Second, blue-green color intensity of artificial model eggs had a significant positive influence on paternal feeding effort. Finally, we found a negative relationship between the effect of experimental food supply on nestling immunocompetence and the intensity of blue-green coloration of eggs, suggesting that egg color predicts nutritional conditions that nestlings will experience during development. All these results taken together strongly support a role of sexual selection in the blue-green coloration of spotless starling eggs.
Article
(1) The aim of this paper is to estimate the shape of the curve relating first year survival to nestling weight in individual great tits (Parus major) and to study the causality of this relationship. (2) Data were collected in a mainland and an island population. Nestlings were weighed and sexed in the nest when 2 weeks old. A recapture programme provided data for recapture-rate estimates in the winter. Local survival until next breeding season was estimated by capturing the breeding population. Brood-size manipulation experiments were performed in the mainland population in order to manipulate nestling weights. (3) The relation between local recapture rate and nestling weight was described using logistic regression techniques. The descriptive model included positive weight and negative squared weight regression coefficients, if controlled for year, sex and date. Recapture rate approached zero at weights of c. 70% of the adult body weight. The curves for both populations showed an approximately linear part over a rather long range of weights. At high weights, the curve levelled off in the mainland population and curved down in the island population. (4) Survival from weighing till fledging and recapture rate from fledging till winter were related to nestling weight, but recapture rate from winter till breeding was not. (5) The effect of brood-size manipulation on nestling weight and subsequent recapture rate suggests causality of the recapture rate-nestling weight curve. Additional information from a comparison of the association between recapture rate and nestling weight within and between broods leads to the conclusion that weight does play a causal role in this relationship. Recapture rate-nestling weight curves can thus be estimated from non-experimental data.
Article
We investigate the way in which the opposing forces of starvation and predation combine to influence daily foraging routines in populations of small, wintering birds. To avoid starvation in an unpredictable foraging environment, feeding should take place early in the day to accrue maximal energy reserves. However, a bird attempting to minimize its risk of predation should delay building up its energy reserves until late in the day. We develop dynamic programming models indicating that the compromise between these opposing forces depends upon the nature of environmental stochasticity in food supply. Foraging may peak early in the day and then decrease steadily if (i) interruptions in foraging are not possible, and (ii) the energetic gain from foraging is relatively poor. With a higher energetic gain, or the possibility of interruptions in foraging, a general "bimodal" foraging routine emerges, with peaks of activity around dawn and dusk. The basic bimodal routine reflects an overt compromise between the avoidance of both predation and starvation, with some feeding taking place during both the early and late portions of the day. Such bimodal routines are observed in many small, wintering birds, but explanations for them have typically focused not on starvation-predation trade-offs, but on daily temporal patterns in food availability or temperature. Our models show not only that bimodal routines emerge in static environments, but also that they are robust to time-dependent environmental variation, and may persist, for instance, even if food is maximally available during the middle of the day. There are currently not enough systematic empirical studies of foraging routines to provide a thorough test of our predictions, but available evidence suggests general agreement between theory and observation.
Article
In various animal species individuals differ consistently in their behaviour, often referred to as personality. In several species these personality differences also correlate with differences in social behaviour. This is important as the social environment is a key selection pressure in many animal populations, mediated, for example, via competition or access to social information. Using social network analysis, recent studies have furthered our understanding of the role of personality in the social environment, usually by focusing on swarming or flocking populations. However, social associations in such populations are fundamentally different from those in territorial populations, where individuals meet less frequently and where the costs and benefits of spatial associations differ from those for swarming or flocking species. In this study we therefore tested whether social network position is related to individual differences in exploration behaviour, an established measure of an avian personality trait, using a wild, territorial, personality-typed great tit, Parus major, population. By means of novel, large-scale, automated tracking (Encounternet) we show, while controlling for average territory distance, that slower exploring males had less central social network positions. Yet, slower explorers overall did not travel shorter distances than faster explorers, indicating that a less central social network position was not merely a consequence of lower activity. Finally, males with less central social network positions did not have reduced breeding success compared to males with more central positions. Our results suggest that territorial individuals influence the structuring of their own social environment in relation to their personality. This is relevant, because the establishment of social relations and familiarity with possible competitors is predicted to be important in many territorial populations.
Article
Interspecifically, a reasonable body of evidence supports a trade-off between offspring size and number. However, at the intra-specific level, a whole manner of phenotypic correlations between offspring size and number are observed. These correlations may be predicted when heterogeneity in resource availability, or quality, is considered. Making the assumption that maternal size is a proxy for resource availability, we meta-analytically quantified four phenotypic reproductive correlations within numerous species: 1) maternal size and offspring size, 2) maternal size and offspring number, 3) offspring number and offspring size and 4) offspring number and offspring size after controlling for maternal size. Within species, maternal size showed a positive correlation with both offspring size and number. Despite this consistency, no correlation between offspring size and number was found. After controlling for maternal size, however, offspring size and number showed a significant negative correlation. A phylogenetic component of our analysis accounted for little heterogeneity in the data, suggesting that our findings show remarkable consistency across taxa. Overall, our results support an observable phenotypic trade-off between offspring size and number. However, this analysis also highlights the importance of considering quality when examining trade-offs, a task that is not always straightforward as quality is context dependant. This article is protected by copyright. All rights reserved.
Article
When animals can choose from a range of feeding options, often those options with a higher energetic gain carry a higher risk of predation. This paper analyses the optimal trade-off between food and predation. We are primarily interested in how an animal's decisions and its state change over time. Our models are very general. They can be applied to growth decisions, such as choice of habitat, in which case we might consider how the state variable size changes over an animal's lifetime. Equally our models are applicable to short-term foraging decisions, such as vigilance level, in which case we might consider how energy reserves vary over a day. We concentrate on two cases: (i) the animal must reach a fixed state, its fitness depending on when this is attained; (ii) the animal must survive to a fixed time, its fitness depending on its final state. In case (i) minimization of mortality per unit increase of state is optimal under certain baseline conditions. In case (ii) behaviour is constant over time under baseline conditions (the `Risk-spreading Theorem'). We analyse how these patterns are modified by complicating factors, e.g. time penalties, premature termination of the food supply, stochasticity in food supply or in metabolic expenditure, and state-dependence in the ability to obtain food, in metabolic expenditure and in predation risk. From this analysis we obtain a variety of possible explanations for why an animal should reduce its intake rate over time (i.e. show satiation). We show how earlier work can be viewed as special cases of our results.
Article
1. Understanding age-specific survival in wild animal populations is crucial to the study of population dynamics and is therefore an essential component of several fields including evolution, management and conservation. 2. We present Bayesian survival trajectory analysis (BaSTA), a free open-source software package for estimating age-specific survival from capture–recapture/recovery data under a Bayesian framework. 3. The method copes with low recapture probabilities, unknown ages (e.g. because of left-truncation) and unknown ages at death (e.g. because of right-censoring). It estimates survival and detection parameters as well as the unknown birth and death times (i.e. latent states) while allowing users to test a range of survival models. In addition, the effect of continuous or categorical covariates can be evaluated. 4. This tool facilitates the analysis of age patterns of survival in long-term animal studies and will enable researchers to robustly infer the effect of covariates, even with large amounts of missing data.
Article
A fundamental premise of life-history theory is that organisms that increase current reproductive investment suffer increased mortality. Possibly the most studied life-history phenotypic relationship is the trade-off between parental effort and survival. However, evidence supporting this trade-off is equivocal. Here, we conducted a meta-analysis to test the generality of this tenet. Using experimental studies that manipulated parental effort in birds, we show that (i) the effect of parental effort on survival was similar across species regardless of phylogeny; (ii) individuals that experienced reduced parental effort had similar survival probabilities than control individuals, regardless of sex; and (iii) males that experienced increased parental effort were less likely to survive than control males, whereas females that experienced increased effort were just as likely to survive as control females. Our results suggest that the trade-off between parental effort and survival is more complex than previously assumed. Finally, our study provides recommendations of unexplored avenues of future research into life-history trade-offs.
Article
Proper attachment of radio transmitters to small passerines is critical to success of telemetry studies. Neck loop and wing loop harnesses pose a number of problems. Gluing techniques are better, but are time consuming. A figure-8 harness that slides on over the legs of the bird, with the transmitter resting over the synsacrum, provides a rapid attachment technique that minimizes problems of balance and physical discomfort.
Article
Fitness variations due to natural variation in the size of the 1st clutch and its laying date were estimated using Fisher's reproductive value for both the clutch (Vc) and the parent (Vp). Artificial variation in brood size was introduced and the consequences in terms of reproductive value estimated. Maximal Vc, computed on the basis of natural variation in clutch size, occurred at a clutch size of 15.2, and increased slightly with laying date. Vp increased with natural variation in clutch size and decreased with date. The total reproductive value V (=Vc + Vp) was maximimal at a clutch size of 15.4, substantially higher than the population mean clutch size (9.2). The components of the reproductive value of Vc that were negatively affected by manipulation were the survival of the nestlings and the recruitment rate. Vp was negatively affected only through the probability of having a 2nd clutch. Maximal Vc computed on basis of artificial variation in clutch size, occurred at a clutch size of 10.0, and also increased with date. Vp decreased with artificial variation in clutch size causing the clutch size maximising reproductive value V to shift to a value of 9.4, very close to the population mean clutch size. Most great tits thus produce the number of eggs (9-10) that maximizes their individual fitness, even though those individual birds laying 15 eggs have the highest reproductive value in the population. -Authors
Article
Life-history theory assumes that organisms trade-off current against future reproduction to maximize fitness. Experimental explorations of the costs of reproduction have not yielded a clear understanding of the nature of these costs but rather point to a complex set of allocation possibilities among several physiological functions and behaviors. We investigated how experimentally increased flight costs affected the trade-off between parental investment and self-maintenance in tropical house wrens, which have relatively high annual survival and multiple breeding opportunities per year. We predicted that handicapped wrens would not increase their energy expenditure but instead decrease their effort to rear young in order to maintain their own body condition. Our results largely supported these predictions: handicapped parents decreased their nestling feeding frequency but did neither alter their field metabolic rate (FMR) nor compromise their body condition as measured by basal metabolic rate (BMR) and several measures of innate immune function. Reduced feeding rates did not affect nestling body mass growth but resulted in decreased structural growth (length of tarsus). The latter result can be explained if parents shifted the type of prey brought to offspring or altered the amount of food brought per trip. The experiment-wide positive correlations among FMR, BMR, and feeding frequency are in agreement with the hypothesis that hard work requires elevated levels of BMR. These correlations, in combination with the absence of a handicap treatment effect on FMR or BMR, do not lend support for predictions from studies in the laboratory that birds compensate hard work during the day by lowering their BMR at night. Considering a complex set of allocation possibilities among several physiological functions and behaviors, we conclude that tropical wrens take out the costs of a handicap largely on their offspring quality not on self-maintenance processes.
Article
Avian fat storage is associated with both benefits and costs. Although the benefits of maintaining higher energetic reserves have long been considered, the associated costs have received far less attention. Spatial and temporal patterns of fat storage, together with experimental data, indicate that birds are capable of actively regulating their energetic reserves at levels below physiological or environmental maxima. This regulation implies that fat storage entails a cost. Evidence of potential costs are reviewed and discussed under the following headings: mass-dependent metabolism, mass-dependent predation risk, mass-dependent risk of injury, mass-dependent foraging, pathological costs and reproductive costs. Although the evidence that fat storage is costly is convincing, key empirical data are lacking. We indicate the sorts of data which need to be gathered and suggest ways in which this might be done. We go on to discuss the interaction of these costs and their relevance to between-individual patterns of fat storage and the interpretation of 'condition indices'. Because many of the purported costs of fat storage are dependent upon changes in body mass, or wing loading, our review is also relevant to other phenomena which may involve mass-dependent costs, such as gonadal hypertrophy, transport of food items and primary moult.
R: a language and environment for statistical computing. Vienna (Austria): R Foundation for Statistical Computing
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R Development Core Team. 2008. R: a language and environment for statistical computing. Vienna (Austria): R Foundation for Statistical Computing. ISBN 3-900051-07-0.