Calibration of the sun compass by sunset polarized light patterns in a migratory bird

Summary Migrating birds derive compass information from the sun, stars, geomagnetic field and polarized light, but relatively little is known about how these multiple sources of directional information are integrated into a functional orientation system. We found that migratory warblers exposed to a rotated polarized light pattern at sunset oriented at a constant angle to the axis of polarization. When polarized light cues were eliminated, this shifted orientation was maintained relative to the setting sun. Polarized light patterns, thus, appear to provide a calibration reference for the sun compass in nocturnal migrants, and may also play a role in calibrating other compass systems. Correspondence to: J.B. Phillips     

The role of daytime cues in the development of magnetic orientation in a night-migrating bird

To assess the role of celestial rotation during daytime in the development of the magnetic compass course, pied flycatchers (Ficedula hypoleuca Pallas, Muscicapidae) were handraised in Latvia under various celestial and magnetic conditions. Tests were performed during autumn migration in the local geomagnetic field (50 000 nT, 73° inclination) in the absence of celestial cues. A group of birds that had never seen the sky showed a bimodal preference for the migratory southwest-northeast axis, whereas a second group that had been exposed to the natural sky from sunrise to sunset in the local geomagnetic field showed a unimodal preference for the seasonally appropriate southwesterly direction. A third group that had also been exposed to the daytime sky, but in the absence of magnetic compass information, also oriented bimodally along a southwest-northeast axis. These findings demonstrate that observing celestial rotation during daytime enables birds to choose the right end of the migratory axis for autumn migration at the Latvian test location. This transformation of axial behavior into appropriate migratory orientation, however, requires the birds to have simultaneous access to information on both celestial rotation and the geomagnetic field. Received: 19 September 1997 / Accepted after revision: 22 November 1997     

Migratory orientation of red-eyed vireos,Vireo olivaceus,in relation to energetic condition and ecological context

How and when migrants integrate directional information from different sources may depend not only on the bird’s internal state, including fat stores, but also on the ecological context during passage. We designed experiments to (1) examine the influence of stored fat on the decision to migrate and on the choice of migratory direction and (2) investigate how the integration of orientation cue information is tied to energetic status in relation to migration across an ecological barrier. Migratory orientation of red-eyed vireos (Vireo olivaceus) at twilight was recorded using two different techniques, orientation cage experiments and free-flight release tests, during both fall and spring migration. During fall migration, the amount of stored fat proved decisive for directional selections of the vireos. Fat birds chose directions in accordance with migration across the Gulf of Mexico. Lean birds oriented either parallel to the coast line (cage tests) or moved inland (free-flight releases). Whereas only fat birds showed significant responses to experimental deflections of the geomagnetic field, lean birds displayed a tendency to shift their activity in the expected direction, making it difficult to evaluate the prediction that use of the magnetic compass is context dependent. Fat loads also had a significant effect on the decision to migrate, i.e., fat individuals were more likely to embark on migration than were lean birds (true for both cage and release experiments). During spring migration, a majority of experimental subjects were classified as lean, following their arrival after crossing the Gulf of Mexico, and oriented in seasonally appropriate directions. The vireos also showed significant responses to experimental deflections of the geomagnetic field regardless of their energetic status. Free-flight release experiments during spring migration revealed a significant difference in mean directions between clear sky and overcast tests. The difference may indicate a compensatory response to wind drift or possibly a need for celestial cues to calibrate the magnetic compass. Finally, this is the first demonstration of magnetic compass orientation in a North American vireo. Received: 15 December 1995/Accepted after revision: 24 March 1996     

Do bearing magnets affect the extent of deflection in clock-shifted homing pigeons?

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.Communicated by W. Wiltschko     

Pigeon homing: sun compass use in the southern hemisphere

Although the sun compass of birds is based on learning the sun's arc during development, it was unclear whether birds can use the sun when its apparent movement is reversed, in particular, whether northern birds that have been introduced into the southern hemisphere can use the southern sun. To answer this question, clock-shift experiments were performed with local homing pigeons in Auckland, New Zealand (37°S). In three fast-shift tests and two slow-shift tests, the experimental birds showed deflections from the untreated controls that were the mirror images of those observed in the northern hemisphere. These results clearly show that homing pigeons in New Zealand use a sun compass that is adapted to the situation in the southern hemisphere. The learning processes establishing the compensation mechanisms thus appear to be free of constraints concerning the direction of the sun's movement. Differences from recent findings with migratory birds, where the direction of celestial rotation proved of crucial importance for establishing the migratory direction, are discussed: the differences may arise from the different orientation tasks, in particular, from the involvement of innate information in establishing the migratory direction. Received: 13 November 1997 / Accepted after revision: 28 February 1998     

Autumn migratory fuelling: a response to simulated magnetic displacements in juvenile wheatears, Oenanthe oenanthe

Recent experiments exposing migratory birds to altered magnetic fields simulating geographical displacements have shown that the geomagnetic field acts as an external cue affecting migratory fuelling behaviour. This is the first study investigating fuel deposition in relation to geomagnetic cues in long-distance migrants using the western passage of the Mediterranean region. Juvenile wheatears (Oenanthe oenanthe) were exposed to a magnetically simulated autumn migration from southern Sweden to West Africa. Birds displaced parallel to the west of their natural migration route, simulating an unnatural flight over the Atlantic Ocean, increased their fuel deposition compared to birds experiencing a simulated migration along the natural route. These birds, on the other hand, showed relatively low fuel loads in agreement with earlier data on wheatears trapped during stopover. The experimental displacement to the west, corresponding to novel sites in the Atlantic Ocean, led to a simulated longer distance to the wintering area, probably explaining the observed larger fuel loads. Our data verify previous results suggesting that migratory birds use geomagnetic cues for fuelling decisions and, for the first time, show that birds, on their first migration, can use geomagnetic cues to compensate for a displacement outside their normal migratory route, by adjusting fuel deposition.     

Orientation of juvenile barn swallows (<Emphasis Type="Italic">Hirundo rustica</Emphasis>) tested in Emlen funnels during autumn migration

Although hirundines have been used extensively in homing experiments, to date no investigation of their migratory orientation has been carried out, despite the well-known migratory habits of many species of this family. This paper reports on a study of the orientation of the barn swallow (Hirundo rustica), a typical diurnal trans-Saharan migrant. Modified Emlen funnels were used to verify the suitability of this species for cage experiments and investigate the role of visual and magnetic cues during the birds first migratory journey. Juvenile swallows were mist-netted at a roost site in central Italy and then tested in a site 19 km apart. Orientation experiments were performed under four experimental conditions: natural clear sky and simulated overcast, in both local and shifted magnetic fields (magnetic North=geographical West). Under clear sky, the swallows tended to orient phototactically toward the best-lit part of the funnel and failed to respond to the magnetic field shift. Under overcast conditions, they oriented northward and modified their directional choices as expected in response to the shifted magnetic North. On the whole, our data indicate that swallows can use magnetic information for compass orientation. Possible explanations for the northward orientation of birds tested under overcast conditions are discussed.Communicated by W. Wiltschko     

Sunset orientation of robins,Erithacus rubecula,with different fields of sky vision

Summary Migratory orientation of robins (Erithacus rubecula) at sunset was recorded using orientation cages, under clear autumn skies. The aim of the experiments was to examine the importance of different visible sky sections for the orientation of robins. I obtained the following results: (1) Robins tested with the visible sky section limited to 90° around zenith (45° above the horizon) showed a mean orientation that coincided with the average sunset azimuth, with little scatter around the mean angle (Fig. 2). (2) When the birds were allowed a more extensive field of sky vision (maximum 160°), they chose headings on an approximate north-south axis, significantly different from tests with a restricted view of the sky (Fig. 3). (3) Experiments were also performed in which the response of robins to a mirror deflection (about 120° counterclockwise) of visual cues in the lower parts of the sunset sky was examined. The outcome indicated that visual information in the lower part of the sky may be critical for the orientation of robins (Fig. 5). These results, together with recent findings that robins captured and tested at two nearby sites show distinctly different orientation behavior in relation to experimental manipulations of the magnetic field, suggest that priorities among orientation cues may differ depending on the migratory situation encountered.     

The development of sun compass orientation in young homing pigeons

Summary 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.Dedicated to Prof. Dr. F.W. Merkel for his 70th birthday     

Solar and landscape cues as orientation mechanisms in the beach-dwelling beetle Eurynebria complanata (Coleoptera,Carabidae)

The present paper reports on the first orientation experiments conducted on the strand-living beetle Eurynebria complanata (Linnaeus, 1767) (Coleoptera, Carabidae). The experiments were carried out from June to October 1988. Two different populations were used: one inhabiting a Tyrrhenian beach in Italy and the other a beach on the Atlantic coast of France. Response to sun compass cues was demonstrated in each population at the collection site and for the French population after transportation to Italy, where experiments were carried out on a differently oriented beach. Landscape cues were shown to improve the beetle's orientation capacity when these were tested together with a visible sun. Artificial landscapes were also tested under laboratory conditions. Different-sized silhouettes were placed in the four cardinal directions, and these envoked different responses depending on their height. The beetles oriented towards a black silhouette when this subtended an angle of 25°. The results obtained for the two populations are compared and discussed from an eco-ethological point of view.     

Geomagnetic field affects spring migratory direction in a long distance migrant

Night-migrating song birds travel to and from their wintering and breeding areas often separated thousands of kilometers apart and are clearly capable of finding intended goal areas from a distant location. Displacement experiments provide a useful way to highlight orientation and navigational skills in migrants. To investigate which cues birds actually use to compensate for displacement and the exact mechanism of each cue, experiments with manipulation of single cues are required. We conducted a simulated displacement of lesser whitethroats Sylvia curruca on spring migration. Birds were displaced not geographically but in geomagnetic space only, north and south of their breeding area to test whether they incorporate information from the geomagnetic field to find their breeding area. Lesser whitethroats held in southeast Sweden but experiencing a simulated displacement north of their breeding area (Norway) failed to show a consistent direction of orientation, whereas birds displaced south of their breeding area (Czech Republic) exhibited consistent northerly orientation, close to the expected seasonally appropriate direction, after displacement toward the trapping location. The absence of a clear compensatory direction in birds displaced north might be due to unfamiliar magnetic information or lack of sufficient information such as a magnetic gradient when moving around. By isolating one orientation cue, the geomagnetic field, we have been able to show that lesser whitethroats might incorporate geomagnetic field information to determine latitude during spring migration.     

Sunset and the orientation of a nocturnal bird migrant: A mirror experiment

Summary If savannah sparrows, (Passerculus sandwichensis), a North American night migrant, select a migratory heading based upon the setting sun, a shift in the position of that cue should produce a predictable shift in the migrant's nocturnal orientation. I tested this hypothesis by shifting the sunst position with mirrors and by recording the bird's orientation in Emlen funnels. The control group displayed directionality appropriate for spring migration (=342°). The mean heading of experimentals (=272°), which were exposed to a cue-shifted situation, was in the expected westerly direction relative to the control mean (P<0.05, V-test). The setting sun appears to be a sufficient source of directional information for this avian migrant.     

Magnetic compass orientation in the yellow-faced honeyeater,Lichenostomus chrysops,a day migrating bird from Australia

Summary In Australia, the southern populations of the yellow-faced honeyeater, Lichenostomus chrysops (Meliphagidae), perform annual migrations, with routes following the eastern coastline. In order to assess the role of magnetic cues in the migratory orientation of this diurnal migrant, its directional behaviour was recorded in recording cages under natural and experimentally manipulated magnetic-field conditions. During autumn the birds tested indoors in the local geomagnetic field showed a directional change from north initially to northwest later in the season (Fig. 1 a, b), which corresponds well with the general pattern of movement of this species in the field. Deflecting magnetic north to ESE resulted in a clockwise shift of the mean direction by 77° and 71°, respectively (Fig. 1 c, d), while no significant directional tendencies were observed in a magnetic field with a compensated horizontal component (Fig. 1 e, f; see Table 1). In outdoor tests in spring, the birds preferred southerly directions when tested in the local geo-magnetic field. In a magnetic field with a reversed vertical component (i.e. with an inclination pointing down instead of upwards) the birds reversed their directional tendencies and oriented northward (Fig. 2, Table 2). These results clearly show: (1) that yellow-faced honeyeaters can use the magnetic field for direction finding, and (2) that their magnetic compass functions as an inclination compass, as has been shown for several holarctic migrants.Correspondence to: W. Wiltschko     

Corticosterone levels in relation to migratory readiness in red-eyed vireos (<Emphasis Type="Italic">Vireo olivaceus</Emphasis>)

We examined the relationship between plasma levels of corticosterone and the migratory activity and directional preference of red-eyed vireos during fall migration at the northern coast of the Gulf of Mexico. Corticosterone is thought to play a role in physiological and behavioural processes before, during, and after long-distance migratory flights. An increase in corticosterone at the onset of migratory flights can be expected in birds that are energetically prepared to migrate in a seasonally appropriate southerly direction. Red-eyed vireos ( Vireo olivaceus) were tested in orientation cages under clear twilight skies. Just prior to the orientation experiments, blood was sampled to assay baseline corticosterone levels. Average corticosterone level for all birds was 22.8 ng/ml. Red-eyed vireos with higher than average baseline levels of corticosterone were significantly more active in orientation cages compared to birds with lower levels of corticosterone. Moreover, birds with higher than average levels oriented in a southwesterly direction, which is consistent with a trans-Gulf flight, whereas individuals with levels below average showed a NNW mean direction. Although there was no significant difference in baseline levels of corticosterone between fat and lean birds, individual mass loss between capture and test was negatively correlated with corticosterone levels. Results from this study clearly demonstrate that corticosterone influences departure decisions and the choice of direction during migration.     

Re-orientation in clock-shifted homing pigeons subjected to a magnetic disturbance: a study with GPS data loggers

Some authors have proposed that homing pigeons are able to correct the error in orientation following a phase-shift treatment by using the magnetic compass reference. They reported that clock-shifted pigeons bearing magnets display a greater deflection compared to magnetically unmanipulated clock-shifted birds. However, this hypothesis tested by recording pigeons’ vanishing bearings has led to contradictory results. The present study reports pigeons’ tracks recorded with a GPS and shows that clock-shifted pigeons bearing magnets displayed a greater deviation through the whole route compared to the magnetically unmanipulated shifted pigeons. Moreover, the analysis of the tracks shows that the birds belonging to both experimental groups stop in coincidence with their subjective night. When re-starting their journey, the birds corrected the clock-shift induced error in orientation, but the magnetically manipulated pigeons were less efficient in doing so. Our results are consistent with the hypothesis that homing pigeons released from unfamiliar location re-orient after clock shift by using the magnetic compass.     

The relationship between social dominance,subcutaneous fat,and annual survival in wintering white-throated sparrows (Zonotrichia albicollis)

Summary We used multivariate analysis to identify factors correlated with level of subcutaneous fat (a form of stored energy) in a migratory, wintering population of white-throated sparrows. Dominant birds, residents from previous years, and birds residing in certain regions of the study area tended to have high mean fat levels during January and February. On the basis of differences in levels of fat, dominant prior residents could probably survive 50% longer without food than subordinate newcomers. An additional analysis revealed that dominant sparrows returned more frequently to the study area than subordinates, a result that might indicate higher survival. Offprint requests to: W.H. Piper (at the present address)     

Influence of fuel load and weather on timing of nocturnal spring migratory departures in European robins, <Emphasis Type="Italic">Erithacus rubecula</Emphasis>

Time of departure and landing of nocturnal migrants are of great importance for understanding migratory strategy used by birds. It allows us to estimate flying time and hence the distance that migrants cover during a single night. In this paper, I studied the temporal schedule of nocturnal departures of European robins during spring migration. The study was done on the Courish Spit on the Baltic Sea in 1998–2003 by retrapping 51 ringed birds in high mist nets during nocturnal migratory departure. Take-offs of individual birds occurred between the first and tenth hour after sunset (median 176 min after sunset). Departure time was not related to fuel stores at arrival and departure, stopover duration and progress of the season. The results suggest that one reason for temporal variation in take-off time was differential response of European robins with high and low motivation to depart to such triggers as air pressure and its trend. If these parameters reach a certain minimum threshold shortly before sunset, robins with a high migratory motivation take off in the beginning of the night. When air pressure or its trend reaches a maximum, it may trigger to take off later during the night birds with lower initial motivation for departure, including those that have low refuelling efficiency. In regulation of timing of take-offs of robins, an important role is also played by their individual endogenous circadian rhythm of activity which is related to the environment in a complex way.     

The magnetic compass mechanism of birds and its possible association with the shifting course directions of migrants

Summary Many birds of the northern hemisphere shift their migratory course to more southerly directions when moving from northern to southern latitudes. Birds from Central Europe, for example, change their course from SW to S or from SE to S respectively (Fig. 1). This also seems to apply to some other animals.The hypothesis presented here explains the observed shifts in migratory direction on the basis of changes in the parameters of the earth's magnetic field and hence would make a genetic fixation of shifts in the migratory direction unnecessary.To determine the direction of migration birds do not refer to the polarity of the magnetic field but to its dip (=). According to the hypothesis presented here, the birds, however, do not refer to the direction of dip as previously believed but to the individual apparent angle of dip (=), this angle changes depending on the heading of the bird (see Fig. 3 and Eq. 1). Maintaining a species specific or population specific the bird will move in its predetermined migratory direction. Changes in the dip of the earth's magnetic field correspond to changes in latitude. According to the hypothesis with fixed, the migratory direction will change when the dip changes. Given the hypothesis and the parameters of the earth's field theoretical migratory paths of birds between summer and winter quarters may be calculated (Figs. 8–11). The calculated tracks and the actually observed migratory routes agree well. This is also confirmed by radar and other observations of migratory directions in areas of different dip angles (Fig. 13). Displacing migrating birds to areas of smaller dip angles (= lower magnetic latitudes) results in predeterminable shifts in the birds migratory direction (Figs. 5, 6). The hypothesis also accounts for the so far unexplained orientation behaviour of transequatorial migrants under the magnetic equator.A very simple model of this hypothetical compass mechanism may be based on the assumption of the sensor axis is supposed to correspond to the apparent angle of dip when moving in the migratory direction. In this position the difference between the apparent angle of dip and the angle of the sensor is zero. Any change in the direction of movement, however, will result in a difference leading to a response of an assumed receptor. When maintaining the zero difference the bird invariably sticks to its migratory course. The proposed mechanism is a null instrument unaffected by changes in field intensity and not depending on the measurement of absolute values.     

Homing behaviour in the sleepy lizard (Tiliqua rugosa): the role of visual cues and the parietal eye

To investigate how visual cues are integrated into a navigational strategy for homing in the Australian sleepy lizard (Tiliqua rugosa), lizards were displaced beyond their home range, either with full access to visual cues or with no access to visual cues during the displacement. Homeward orientation was significantly worse when lizards were denied visual cues during the displacement than when they were not. However when lizards were displaced with their field of view restricted to the sky, their homeward orientation was equally as good as that of lizards displaced with no visual restriction. These experiments suggest that sleepy lizards use celestial cues to determine the compass bearing of the outward journey, and reverse this bearing to orient in the homeward direction (course reversal). In a subsequent experiment, lizards oriented randomly with respect to home when the parietal eye was entirely covered with a patch during the displacement and return, while control lizards fitted with a sham parietal eye patch were well oriented towards home. In both groups, the lateral eyes were unobstructed and had complete access to visual cues including celestial cues and landmarks. These results suggest that the parietal eye plays a highly significant role in sleepy lizard homing, perhaps mediating a sky polarization compass sense.     

Compensatory behaviour after displacement in migratory birds

Using a meta-analysis approach we re-analysed orientation cage experiments with displaced migrants found in the literature. A rather large proportion of the orientation experiments showed directional shifts after displacements, indicating ability for birds to detect and react on such displacements. There was a clear difference between overcast and experiments where birds had a view of the starry sky. In experiments under a starry sky, the birds compensated the displacements, whilst under overcast unaltered or reverse orientations were generally displayed. This indicates a role for the stars to be involved in detection of the changes in position. Such a role of celestial cues is further stressed by the results of several studies manipulating a planetarium sky. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.