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.     

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     

Inspection of mob-calls as sources of predator information: response of migrant and resident birds in the Neotropics

Migrating animals face numerous mortality risks, such as novel predators with which they may not be accustomed. Most animals can recognize predators innately; however, additional predator information can be collected to enhance familiarity. Because migrating birds rarely participate in mobs, they may seek alternative information sources such as cues provided by other birds that can provide information on predator location, identity, and degree of threat. We predicted that Nearctic–Neotropical migrants (hereafter, “migrants”) would react to vocal antipredator cues (e.g., mob-calls) of species residing in areas through which they migrate. To test this, we conducted experiments in Belize during spring migration, using playbacks of mob-calls of black-capped chickadees (Poecile atricapillus) and blue-gray tanagers (Thraupis episcopus); tanagers are familiar to all birds in Belize; chickadees are novel to residents but familiar to migrants. This also allowed us to assess response to novel and out-of-context antipredator signals. Resident birds did not respond to novel chickadee mob-calls, but did respond to familiar tanager calls. Birds overwintering south of our study area, which were migrating during our study, responded most strongly to chickadee playbacks. Conversely, individuals of species that include our study area in their winter range did not respond to either playback. This is the first evidence that birds react to vocal antipredator cues during migration, which may be a strategy used by migrants to learn about predators. Although residents failed to recognize a foreign cue, migrating birds responded most strongly to the out-of-context chickadee cue, associated with breeding grounds >2,000 km northward. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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     

Clock-shift experiments with Savannah sparrows, Passerculus sandwichensis, at high northern latitudes

Orientation can be difficult for nocturnal bird migrants at high northern latitudes because of the large changes of magnetic declinations, rapid longitudinal time-shifts experienced during a long-distance flight and the invisibility of stars during the polar summer. Both sunset cues as well as geomagnetic cues have been shown to be of great importance in the orientation system of Savannah sparrows, Passerculus sandwichensis. We used clock-shift experiments to investigate whether geomagnetic and sunset cues were used for migratory orientation by wild-caught young Savannah sparrows at high geomagnetic latitudes in Northern Canada. We exposed birds to a 4-h slow clock-shift, expecting a 60° clock-wise shift in orientation after the treatment. Under natural clear skies in the local geomagnetic field, the birds responded by showing a significant axial mean orientation directed towards the position of the setting sun in the NW and towards their preferred migratory direction in the SE. After exposure to the clock-shift for 6 days and nights the birds showed a clear response to the treatment and shifted significantly towards NNE. Birds that first oriented towards NW in the experiments before clock-shift tended to shift clock-wise, thus reacted to the clock-shift in the expected way. The reaction of the individual birds that originally oriented towards SE seems to vary. In summary, our birds did not select a constant angle (menotaxis) in relation to the sun's position during the experiments, but presumably were affected by the sun showing phototaxis or followed their magnetic compass. Possible explanations of the unexpected experimental results are discussed. Electronic Publication     

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     

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.     

Is there a different response to winds during migration between soaring and flapping raptors? An example with the Montagu’s harrier and the lesser kestrel

During migrations, birds have to cope with varying meteorological conditions, which shape their migratory routes and affect their performance. Amongst these, wind is the main meteorological agent influencing behaviour of birds in their migration journeys. Here we analyze the effect of winds during migrations of adult individuals of two raptor species tracked with satellite telemetry, the Montagu’s harrier (Circus pygargus) and the lesser kestrel (Falco naumanni). While harriers use mostly soaring flight, kestrels principally use flapping flight and thus, wind can differently affect these birds. We found that both forward and perpendicular winds significantly affected the movements of the Montagu’s harrier, which were drifted from their intended direction but also took advantage of tailwinds. On the contrary, lesser kestrels moved more regardless of forward winds, despite they were highly drifted by crosswinds. Our results also support that the drifting effect of winds at the onset of the spring migration may explain the loop migration observed for both species, with birds compensating later the effect of crosswinds to arrive to their breeding areas. Results presented here illustrate how winds can differently affect migrating birds according to their flight modes.     

Orientation of the pied flycatcher Ficedula hypoleuca: cue-conflict experiments during spring migration

Hierarchical relationships among different compass systems in long-distance migrants are still a matter for discussion because different studies have led to highly variable and apparently contradictory results. We carried out cue-conflict experiments during spring migration on pied flycatchers Ficedula hypoleuca (Passeriformes, Muscicapidae). Birds were exposed to a conflict between celestial and magnetic information by altering the polarized light pattern or magnetic field. The polarization pattern was shifted (±90°) with filters, whereas the magnetic field was altered (+90°) through Helmholtz coils. Birds were tested in modified Emlen funnels both before and after the cue conflict; during the tests, only the natural magnetic field was available. This protocol was designed to test whether the experimental birds recalibrated their magnetic compass on the directional information derived from the light polarization pattern when the region near the horizon was visible during the conflict. Contrary to this expectation, we did not record any significant shift in magnetic orientation after one or repeated exposures to the cue conflict. Our results support earlier studies, which suggest that the magnetic field is the primary compass cue during the migratory period.     

Northern magnetic displacements trigger endogenous fuelling responses in a naive bird migrant

In a previous study, we found that juvenile northern wheatears (Oenanthe oenanthe) exposed to a magnetic displacement to the west of their natural migration route increased their body mass. The total intensity and inclination used for the western displacement may also have been interpreted as northern compared to the experimental site (stronger total field intensity and steeper inclination angle). In order to investigate whether the fuelling response was a response to an unexpected magnetic field or specific to the northern magnetic field, we conducted a new experiment. Juvenile wheatears from the same study population were magnetically displaced to southwestern magnetic fields, exposing the birds to unexpected magnetic combinations, but eliminating the possible effect of a northern magnetic field. A control group was kept in the local geomagnetic field in Sweden for comparison. There was no difference in body mass increase between treatments, suggesting that the fuelling response previously found was not a simple response to an unexpected magnetic field, but rather a specific response to the northern magnetic field. Juvenile wheatears may have developed a fuelling response to northern magnetic fields in order to enable a successful flight towards the migration goal.     

Exaggerated orientation scatter of nocturnal passerine migrants close to breeding grounds: comparisons between seasons and latitudes

Using tracking radars, we investigated the variability of flight directions of long-distance nocturnal passerine migrants across seasons (spring versus autumn migration) and sites at the southern (56° N) and northern (68° N) ends of the Scandinavian Peninsula (Lund versus Abisko). Whilst most migrants at Lund are on passage to and from breeding sites in Fennoscandia, the majority of the migrants at Abisko are close to their breeding sites, and migration at Abisko thus to a large degree reflects initial departure from breeding sites (autumn) or final approach to breeding destinations (spring). The radar data were used to test predictions about differences in orientation and wind drift effects between adult and juvenile birds (a large proportion of autumn migrants consists of juvenile birds on their first journey), between situations far away from or near the goals and between different phases of migration (initial departure, en route passage, final approach to goal). The concentrations (both total and within-night concentrations) of flight directions differed significantly between seasons as well as sites, with the highest concentration at Lund in spring (mean vector length of track directions, r = 0.79) and lowest at Abisko during spring (r = 0.35). Partial wind drift and partial compensation were recorded at Lund, with a similar effect size in spring and autumn, whilst possible wind drift effects at Abisko were obscured by the large directional scatter at this site. The results from Lund support the prediction that the high proportion of juveniles in autumn contributes to increase the directional scatter during this season, whilst there was no support for predictions of differential wind drift effects between seasons and situations with different goal distances. The most striking and surprising result was the exceedingly large scatter of flight directions at Abisko, particularly in spring. We suggest that such an exaggerated scatter may be associated with final approach orientation, where migrants reach their specific goals from all various directions by final navigation within a more wide-ranging goal region. The larger scatter of autumn flight directions at Abisko compared to Lund may be due to exploratory flights in variable directions being more common at initial departure from breeding sites than later during migratory passage. These surprising results highlight the importance of studying and analysing orientation during final approach to (and initial departure from) migratory goals for understanding the orientation systems of migratory birds.     

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     

Movements and wintering areas of breeding age Thick-billed Murre <Emphasis Type="Italic">Uria lomvia</Emphasis> from two colonies in Nunavut,Canada

The non-breeding movements of marine birds were poorly known until recently, but this information is essential to understanding the risk to different geographical populations from events on the wintering grounds. We tracked the migration routes and wintering areas of Thick-billed Murre Uria lomvia from two breeding colonies in eastern Canada: Coats Island in northern Hudson Bay and The Minarets, Baffin Island, during the period August 2007–May 2008 using geolocation loggers. Birds from The Minarets moved south rapidly post-breeding and wintered principally off Newfoundland and southern Labrador, or between Newfoundland and southern Greenland, remaining south of 55°N until at least the spring equinox. Those from Coats Island remained in Hudson Bay until at least mid-November, after which they moved rapidly through Hudson Strait to winter in southern Davis Strait and the northern Labrador Sea, mostly north of 55°N. Many individuals stayed throughout the winter in areas of heavy ice cover. Adults from the two colonies appear to be completely segregated in winter and those from Coats Island probably did not enter the area of the winter hunt in Newfoundland. Unexpectedly, some birds from The Minarets wintered in waters beyond the continental slope and outside the distribution of pack ice, demonstrating that particular individuals can be wholly pelagic throughout the winter. Coats Island birds returned through Hudson Strait as soon as open water areas became available in spring. Their sojourn in Hudson Bay coincided very closely with the occurrence of areas with <90% ice cover. In spite of the relatively large error in positions obtained from geolocation loggers, our results demonstrated the value of these devices by uncovering a number of previously unknown aspects of Thick-billed Murre non-breeding ecology in the Northwest Atlantic. Comparison of the non-breeding ecology based on SST experienced in winter show that the winter niche is broader than hitherto assumed, demonstrating that separate populations may experience different selection in the face of climate change.     

Satellite tracking of migrating loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) displaced in the open sea

Loggerhead turtles (Caretta caretta) are known to migrate towards fixed, individually-specific residential feeding grounds. To study their spatial behaviour and their navigational ability, five loggerheads nesting in South Africa were captured when about to start their postnesting migration and tracked by satellite after having been displaced from their usual migratory route. The first turtle, released south of Madagascar about 1,148 km from the capture site, moved west up to mainland Africa and then reached her feeding grounds by following the coast. A second turtle, released farther away (2,140 km) close to La Réunion Island, stopped for some time on the Madagascar east coast, then turned southwards to round the island and regain the African mainland in the northwest, without however allowing us to establish the location of her residential grounds. Three other turtles were released off the Tanzanian coast, 2,193 km north of their nesting area, at the northern edge of the distribution of the feeding grounds along the African coast. All of them headed north, and one turtle found her residential grounds located north of the release site. The other two females started long-distance oceanic wanderings in which they crossed nearly the entire Indian Ocean, apparently being transported by the sea currents of the region. We conclude that adult loggerhead turtles are apparently unable to compensate for the displacement and can return to a pelagic life style characteristic of juvenile turtles. These findings suggest that South African loggerheads rely on simple orientation mechanisms, such as the use of the coastline, as a guide, and compass orientation, possibly integrated by spatiotemporal programmes and/or acquired maps of familiar sites.Communicated by R. Cattaneo-Vietti, Genova     

Can pigeons be fooled about the actual release site position by presenting them information from another site?

Summary The aim of the experiment was to test the hypothesis that pigeons depend on route- and/or site-specific airborne parameters to establish their position relative to the loft. Pigeons were transported to the release site with free access to the environmental air. They were then enclosed in large airtight containers filled with air from the release site and either transported via the loft to a release site in the opposite direction with respect to the loft (experimental birds) or via the loft back to the same site (control birds). Before release the pigeons (still inside the containers) were made anosmic by a local anesthetic applied to the olfactory membranes through the nostrils. Vanishing bearings of experimental birds were on the average in a direction opposite to home while the vanishing bearings of control birds were homeward oriented. For this initial orientation the pigeons seem to rely on airborne cues obtained at a site where they last had access to ambient air or cues obtained from the air inside the sealed containers. Irrespective of the treatment the bearings pooled with respect to north still show a residual orientation to the NE. There was no difference either in the homing speeds or in the homing times of anosmic control and experimental pigeons. We therefore have to assume further mechanisms guiding the pigeons home in addition to a possibly olfactory one.     

Homing behavior of wood thrushes (Hylocichla mustelina)

Summary Using the wood thrush (Hylocichla mustelina) as an example of a typical nocturnal migrant, we employed radio telemetry to follow breeding birds during the course of homing movements following displacements. Seven thrushes were displaced over distances of 6.5 to 17.3 km in a variety of directions from their nesting territories. The thrushes moved in a series of short flights (mean = 2.1 km) performed primarily at dawn. Consequently, the birds took several days to home from even these relatively short displacements. Thrushes flew under clear and solid overcast skies and even in light rain. The pooled individual flights of the birds were significantly oriented in the homeward direction (Fig. 2). Their orientation relative to home did not improve significantly as they progressed toward the goal. Three of the birds were documented to have returned to their home territories. The detailed tracks of the birds preclude the possibility that they homed by random search.     

Orientation in captive migratory and sedentary Australian silvereyes <Emphasis Type="Italic">Zosterops lateralis</Emphasis> (Zosteropidae)

The orientation of two closely related subspecies of Australian silvereye Zosterops lateralis was studied in captivity over 14 months. Migratory silvereyes Z. lateralis lateralis showed significant directional preferences during the spring and autumn migration periods and also displayed orientated behaviour during the breeding period. In contrast, the non-migratory subspecies Z. lateralis familiaris did not display any significant directional preferences at any time. This is the first time that the orientation behaviour of a migratory and non-migratory subspecies has been compared over the duration of an annual cycle, both during and outside the migratory periods. The results suggest that migratory silvereyes possess an endogenous program determining the timing and direction of autumn and spring migration and that this program is unique to the migratory subspecies. This is also the first comparison of this nature on southern hemisphere birds and demonstrates that the migratory behaviour of southern hemisphere migrants may be more similar to that of northern hemisphere migrants than previously thought.Submitted to Behavioral Ecology and Sociobiology: 23 Jan 2006.     

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     

The avoidance response of fathead minnows to chemical alarm cues: understanding the effects of donor gender and breeding condition

Summary. All animals are vulnerable to predation at some point in their lives and consequently prey organisms often develop effective risk assessment systems. For many aquatic species predation risk assessment occurs through the use of olfactory cues, including predator odours and alarm cues from damaged or disturbed conspecifics. When aquatic species encounter conspecific alarm cues they may respond, or not, based on specific information including cue concentration, health and size of the conspecific donor and potentially the gender and breeding condition of the donor. Previous laboratory studies have demonstrated that fathead minnows (Pimephales promelas) fail to respond to the skin extracts of breeding male minnows. The purpose of the current study was to verify these early laboratory findings in the field as well as to further investigate the effect of female reproductive state and donor gender on the response of minnows to damage-release alarm cues. Our results indicate that male breeding condition has a significant effect on how minnows will respond to conspecific cues. Minnows showed avoidance of cues of female minnows and male minnows not in breeding condition, in comparison to cues of breeding male minnows and cues of male and female swordtails. Neither the gender of non-breeding minnows nor the reproductive state of female minnows influenced the avoidance of minnows to alarm cues.     

Orientation mechanisms in migrating European silver eel (Anguilla anguilla): Temperature and olfaction

The migration pattern of silver eel (Anguilla anguilla L.) in the Baltic is well documented from many tagging experiments. This particular investigation differed from previous taggings in that the background of all the eels used was the same. They came from a 1980 stocking programme and had been imported from France as glass eels. When migrating these stocked eels missed the outlet of the Baltic and continued in a south-westerly direction. They were still reported in this southern-most area more than 4 yr later. Indigenous eels turn northwards after passing the south of Sweden and leave the Baltic through the narrow Sound. Since the formerly stocked eels had never been in the Baltic, the absence of an imprinted olfaction cue may explain their orientation failure. Although lacking an imprinted olfaction cue eels were recaptured mainly in a very restricted area in the southwestern Baltic. This reflects another cue, genetic or imprinted during the Atlantic journey. This second cue, temperate, can also serve as the trigger which causes eels to stop migrating and to initiate spawning. The enclosed Baltic acts as a giant trap for eels from the huge stocking programmes undertaken there. As a consequence stocked eels probably contribute little to spawning stocks in the Sargasso Sea which may have contributed to the decrease in abundance of glass eels reported from western Europe in recent decades.