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When does bearing magnets affect the size of deflection in clock-shifted homing pigeons?
Abstract
Pigeons whose internal clock is shifted by 6h show deflections from the direction of untreated controls, yet these deflections are often smaller than predicted. Magnets temporarily disabling the magnetic compass increased these the deflections significantly (R. Wiltschko and Wiltschko 2001), indicating a compromise between sun compass and magnetic compass. – Recently, Ioalé et al. (2006) claim that they could not replicate our findings. The reason lies in a difference in the behavior of the clock-shifted pigeons without magnets: in the study of Ioalè et al. (2006), their deflections was already almost as large as that of our pigeons carrying magnets. This difference is probably caused by the limited experience of the pigeons of Ioalè et al. (2006): Their birds, in contrast to ours, had not used their sun’ compass during extended homing flights at various times of the year and, not having been faced with the necessity to compensate the saisonal changes of the sun’s arc, gave the sun compass more weight than our birds did.
... However, a large body of evidence both in field releases and arena experiments has demonstrated that pigeons with access to the magnetic compass and sun compass reliably show deviation under clock-shift, showing that the sun compass is used preferentially to the magnetic compass, even when a cue conflict occurs [15,[17][18][19][23][24][25][38][39][40][41][42]. Some studies have shown that the magnetic compass may be involved in re-orientation following clock-shift during flight over unfamiliar areas, after several kilometres of flights either at vanishing to a lesser and variable extent or well beyond the release site area to a greater extent [43][44][45]. GPS-tracking data showed that a complete re-orientation of the birds occurred after several hours and often after the subjective night possibly due to several factors [45]. This suggests that it is unlikely to have occurred in our arena setting where the test sessions occurred over a narrow time frame [17][18][19]24,25,38]. ...
Functional lateralisation in the avian visual system can be easily studied by testing monocularly occluded birds. The sun compass is a critical source of navigational information in birds, but studies of visual asymmetry have focussed on cues in a laboratory rather than a natural setting. We investigate functional lateralisation of sun compass use in the visual system of homing pigeons trained to locate food in an outdoor octagonal arena, with a coloured beacon in each sector and a view of the sun. The arena was rotated to introduce a cue conflict, and the experimental groups, a binocular treatment and two monocular treatments, were tested for their directional choice. We found no significant difference in test orientation between the treatments, with all groups showing evidence of both sun compass and beacon use, suggesting no complete functional lateralisation of sun compass use within the visual system. However, reduced directional consistency of binocular vs. monocular birds may reveal a conflict between the two hemispheres in a cue conflict condition. Birds using the right hemisphere were more likely to choose the intermediate sector between the training sector and the shifted training beacon, suggesting a possible asymmetry in favour of the left eye/right hemisphere (LE/RH) when integrating different cues.
... Wiltschko and Wiltschko, 2001). Recently, the magnetic compass has been described in chickens, indicating the potentially important role of magnetic orientation even in nonmigratory species (Freire et al., 2005, 2008; Wiltschko et al., 2007). In pigeons, the sun compass is preferentially used over the magnetic compass, especially when the sun is visible, but it is not established whether chickens exhibit a similar preference. ...
Domestic chicks are able to find a food goal at different times of day, with the sun as the only consistent visual cue. This suggests that domestic chickens may use the sun as a time-compensated compass, rather than as a beacon. An alternative explanation is that the birds might use the earth's magnetic field. In this study, we investigated the role of the sun compass in a spatial orientation task using a clock-shift procedure. Furthermore, we investigated whether domestic chickens use magnetic compass information when tested under sunny conditions.
Recent research into the navigational strategies of homing pigeons, Columba livia, in the familiar area has highlighted the phenomenon of route fidelity: birds forming idiosyncratic flight paths to which they are loyal over multiple releases from the same site, and even returning to this path when released from a nearby unfamiliar location. Such results highlight the potential importance of visual landmark cues in the homing process. However, not all birds have been found to produce idiosyncratic routes or show this route-joining behaviour. Here we used birds with and without flight experience to study the formation of idiosyncratic routes when released repeatedly from a single location, followed by two off-route releases with differing topography to see how flight experience and local landmark features can influence navigational strategy in the familiar area. We found that, over the course of 20 sequential releases, birds with greater flight experience tended to form idiosyncratic routes whereas less experienced birds did not show this tendency. When released from nearby sites (from which the birds had not previously been released), a range of navigational strategies were seen, including flying parallel to the learned route (suggestive of a learned compass direction), a direct flight path towards home (again indicative of compass use), rejoining the learned route and following the coastline. These latter strategies are suggestive of landmark usage. Analysis using time lag embedding was also used to assess the off-route releases, and the short-term correlation dimension values produced were also indicative of strategies using one or two factors (landmarks, compass or a combination of these two). Individual birds often showed different strategies at different sites, suggesting that the use of different navigational cues is highly flexible and situationally dependent.
In adult homing pigeons, the deflections induced by a 6-h clock-shift are often markedly smaller than predicted on the basis of the difference in sun azimuth. To look for possible reasons, we performed clock-shift experiments with adult pigeons, releasing birds with small bar magnets that interfered with their magnetic compass. Initial orientation and homing performance of control birds living in the natural photoperiod were not affected by the magnets in any way. Otherwise untreated pigeons whose internal clock had been reset by 6 h showed deflections that were about 60% of the predicted size. Brass bars did not affect the behavior of the clock-shifted birds. Magnets, however, increased the deflection significantly to about 90% of the predicted size; in the majority of releases, the confidence interval of the clock-shifted birds with magnets included the expected direction. These findings suggest that the reduced deflections observed in adult pigeons are to be attributed to the pigeons' use of their magnetic compass: when pigeons locate their home course, they seem to combine directional information from the sun compass with information from the geomagnetic field.
This analysis is based on 103 releases with 6-h clock-shifted pigeons of various ages and experiences. Resetting the internal clock normally leads to a significant change in initial orientation; however, in half of the cases, the induced deflections are significantly smaller than predicted by the sun compass hypothesis. The relative size of the deflections decreases with increasing age and experience (Fig. 3). Only young pigeons with limited experience respond as expected, while old birds show deflections which are, on the average, only slightly more than half of the predicted size, except at extremely familiar sites (Table 2). There is no difference between fast and slow shifts (Fig. 4). It is not possible to clearly specify under what circumstances smaller deflections occur; previous clock-shifts (Fig. 5), familiarity with the release site (Table 4) and duration of the shifting procedure (Table 5) do not seem to be the reasons. Clock-shifting also tends to decrease the vector lengths and has a marked effect on homing performance (Table 7). Nevertheless, considerable numbers of clock-shifted birds return on the day of release before their internal clock has begun to be reset back to normal. The general role of the sun compass in bird orientation is considered and theoretical implications of our findings are discussed in view of the map and compass-model and the possibility that an alternative, non-time-compensating compass is used in parallel with the sun compass.
Clock-shifted homing pigeons were tracked from familiar sites 17.1 km and 23.5 km from the home loft in Pisa, Italy, using an on-board route recorder. At the first release site, north of home, the majority of clock-shifted birds had relatively straight tracks comparable with those of control birds. At the second release site, south of home, the clock-shifted birds deflected in the direction predicted for the degree of clock shift, with many birds travelling some distance in the wrong direction before correcting their course. The possible role of large-scale terrain features in homing pigeon navigation is discussed.
To use the sun for direction finding, the birds must compensate for the sun's apparent movement in the course of the day. A method of demonstrating the use of the sun compass is based on shifting the bird's internal clock, which results in a typical deviation of their vanishing bearings (Fig. 1). The specific relation between sun azimut, time and geographic direction depends on geographic latitude (Fig. 2) and varies in the course of the seasons (Fig. 3). The knowledge of this relationship which is the basis for compensation of the sun's movements, is aquired by experience (Fig. 4). The respective learning processes take place when the young birds obtain some flying experience; they must observe the sun during the various times of the day. The magnetic field can serve as a reference system to judge the sun's position.
To analyse the nature of these learning processes, young pigeons were moved for one to two weeks into a small loft (Fig. 5) surrounded by Helmholtz coils to alter the ambient magnetic field. From an aviary, the birds saw the sun in an abnormal directional relationship to the magnetic field. In the following releases, old, experienced pigeons were not affected by this treatment. Young birds of the age when the sun compass normally develops, however, showed significant deviations to the direction to which magnetic north had been turned (Fig. 6, 7). This difference between young and adult pigeons suggests a sensitive period for the development of the sun compass. A series of experiments in which adult pigeons were clockshifted and kept under these conditions for more than two months (Fig. 8) clearly demonstrated that the compensation mechanisms for the sun's movements, once learned, are not fixed, but can be adapted to altered situations (Fig. 9, 10).
Thus the sun compass proved to be a flexible mechanism that can be adjusted not only to the changes in the sun's arc in the course of the year, but also to the greater changes experienced by migratory birds between breeding ground and wintering area. The processes leading to the establishment of the sun compass and possible similarities to the development of the star compass and the navigational “map” are discussed: All three mechanisms seem to be based on learning processes that are in part imprinting-like, involving a sensitive period where the mechanisms per se have to be established. The specific reaction to the external factors, however, is not fixed, but can be adapted to the changes in the environment.
To test whether the sun compass of pigeons is calibrated by the magnetic field, a group of young pigeons was raised in an altered magnetic field in which magnetic north was turned ca. 65o (in 1974 and 1975) and 120o (in 1980) clockwise. They could see the sun only in an abnormal relation to the magnetic field, since they were released for exercise flights or training flock tosses only when the sky was totally overcast.
On their first flight in sunshine these experimental birds deviated clockwise from the mean of their controls; the amount of this deviation was, however, only about half of the shift in magnetic north. On their second flight in sunshine the clock-wise deviation changed to counterclockwise. This change occurred after an exercise flight in sunshine as well as after a homing flight. On later flights in sunshine the differences in orientation between experimentals and controls seemed to disappear.
These findings indicate that the magnetic compass is involved in the learning process to establish the sun compass, but the relation between the two systems is more complex than the calibration hypothesis assumed.
A series of clock-shift experiments with young homing pigeons of various ages was performed to determine at what age they normally learn sun compass orientation. The response of untrained pigeons to shifting of their internal clock seems to depend on their age. When the clock-shifted birds were tested at an age of 11 weeks and younger, their departure bearings did not differ significantly from those of controls (Fig. 1, diagrams on the right); in tests with birds 12 weeks and older the characteristic deviation indicating the use of the sun compass was observed (Figs. 2 and 3). Birds that had participated in a short training program, however, used the sun compass at 8 weeks, the earliest age tested (Fig. 1, diagrams on the left). These findings show that the time of development of the sun compass strongly depends on flying experience. Within the first months of a bird's life, it seems to take place after the bird has been confronted with the need to orient, either spontaneously during extended exercise flights around its loft or imposed by training releases.
The departure bearings of the very young, inexperienced birds that did not rely on the sun compass, however, were already oriented homeward. This indicates that the ability to navigate develops independently of the sun compass, before the sun compass is learned.
To analyze the navigational strategy of homing pigeons at familiar sites in view of a possible role of local landmarks, two groups of pigeons—one familiar to the release site, the other unfamiliar—were released with their internal clock shifted 6h fast, with untreated birds of both groups serving as controls. The two groups showed median deflections of 67% and 57%, respectively, of the expected size, with no consistent difference in the size of the deflection between familiar and unfamiliar birds. This clearly shows that familiarity with the release site and with the local landscape features does not affect the size of the deflections induced by clock-shifting. Obviously, pigeons familiar with the release site do not change their navigational strategy, but still continue to determine their home course solely as a compass course. General problems with orientation by landmarks are discussed; however, landmarks may help birds to recognize a site and recall the respective course.
When released after clock-shift, homing pigeons fail to orient towards the home direction but display a consistent deflection
of their initial orientation due to the difference between the real sun azimuth and the computed azimuth according to the
subjective time of each single bird. It has been reported that the size of the observed deflection is frequently smaller than
expected and a discussion on the possible factors affecting the size of deflection has emerged. Some authors have proposed
that the major factor in reducing the deflection after clock-shift is the simultaneous use of both the magnetic and the sun
compasses, giving true and erroneous information, respectively, about the home direction. Therefore, a magnetic disturbance,
by impeding the use of the geomagnetic information in determining the home direction, is presumed to increase the size of
the deflection up to the levels of the expectation. To test this hypothesis, we released three groups of clock-shifted birds
from unfamiliar locations (unmanipulated pigeons, pigeons bearing magnets on their head, and pigeons bearing magnets on their
back) together with a group of unshifted control birds. As no difference in the orientation of the three groups emerged, we
were not able to confirm the hypothesis of the role of the magnetic compass in reducing the expected deflection after clock-shift.