Roland Sandberg

Why do migrating robins, Erithacus rubecula, captured at two nearby stop-over sites orient differently?

Abstract The orientation of robins captured during autumn and spring migration at two different sites, Falsterbo and Ottenby, in southern Sweden was investigated by cage experiments during the twilight period after sunset. The robins were tested under clear skies with skylight from sunset visible, and under simulated total overcast. The robins from the two sites differed in orientation, especially during autumn migration. While robins from Ottenby generally oriented in their expected migratory direction, the birds from Falsterbo under clear skies oriented towards the sunset direction with a narrow scatter in individual mean headings. Under simulated total overcast the robins from Falsterbo perferred northerly directions in autumn. Short-distance recoveries, one or only a few days after ringing, show that robins in autumn regularly fly 20–80 km from Falsterbo on northerly courses, indicating that they have temporarily reoriented from their normal migratory direction when confronted with the Baltic Sea. In contrast, most robins arrive at Ottenby by extensive flights across the Baltic Sea, and rapidly continue their sea crossing in the normal migratory directions. Mean fat deposits in autumn robins were significantly larger at Ottenby than at Falsterbo. These results indicate that migrating birds may show markedly different orientational dispositions depending on body condition and on their situation with respect to preceding and impending migration over land and sea, respectively.

Shifted magnetic fields lead to deflected and axial orientation of migrating robins, Erithacus rubecula, at sunset

Abstract The migratory orientation of the robin was tested in shifted magnetic fields during the twilight period after sunset, under clear skies and under simulated total overcast. The horizontal direction of the geomagnetic field was shifted 90° to the right or left in relation to the local magnetic field, without changing either the intensity of the field or its angle of inclination. Experiments were conducted during both spring and autumn, with robins captured as passage migrants at the Falsterbo and Ottenby bird observatories in southern Sweden as test subjects. Generally, the orientation of robins was affected by magnetic shifts compared to controls tested in the natural geomagnetic field. Autumn birds from the two capture sites differed in their responses, probably because of different migratory dispositions and body conditions. The robins most often changed their orientation to maintain their typical axis of migration relative to the shifted magnetic fields. However, preferred directions in relation to the shifted magnetic fields were frequently reverse from normal, or axial rather than unimodal. These results disagree with suggested mechanisms for orientation by visual sunset cues and with the proposed basis of magnetic orientation. They do, however, demonstrate that the geomagnetic field is involved in the sunset orientation of robins, probably in combination with additional visual or non-visual cues that contribute to establish magnetic polarity.

Magnetic information calibrates celestial cues during migration

Migratory birds use celestial and geomagnetic directional information to orient on their way between breeding and wintering areas. Cue-conflict experiments involving these two orientation cue systems have shown that directional information can be transferred from one system to the other by calibration. We designed experiments with four species of North American songbirds to: (1) examine whether these species calibrate orientation information from one system to the other; and (2) determine whether there are species-specific differences in calibration. Migratory orientation was recorded with two different techniques, cage tests and free-flight release tests, during autumn migration. Cage tests at dusk in the local geomagnetic field revealed species-specific differences: red-eyed vireo, Vireo olivaceus, and northern waterthrush, Seiurus noveboracensis, selected seasonally appropriate southerly directions whereas indigo bunting, Passerina cyanea, and grey catbird, Dumetella carolinensis, oriented towards the sunset direction. When tested in deflected magnetic fields, vireos and waterthrushes responded by shifting their orientation according to the deflection of the magnetic field, but buntings and catbirds failed to show any response to the treatment. In release tests, all four species showed that they had recalibrated their star compass on the basis of the magnetic field they had just experienced in the cage tests. Since release tests were done in the local geomagnetic field it seems clear that once the migratory direction is determined, most likely during the twilight period, the birds use their recalibrated star compass for orientation at departure. Copyright 2000 The Association for the Study of Animal Behaviour.

Migratory Orientation of Free-Flying Robins Erithacus rubecula and Pied Flycatchers Ficedula hypoleuca: Release Experiments

Release experiments were conducted to examine a number of questions concerning the orientation behaviour of free-flying birds. (1) We found distinct differences in the orientation performance of Robins released under clear and overcast skies, respectively, during autumn migration. While Robins oriented in a seasonally appropriate direction under clear skies, they selected a northwesterly mean direction under overcast, pointing to the importance of visual celestial cues during autumn migration. There was no significant difference in orientation under clear and overcast skies in releases performed during spring migration. Pied Flycatchers failed to orient in their expected migratory direction when released under clear autumn skies, probably because of strong winds. (2) The amount of stored fuel reserves had a pronounced influence on the decision to take off on migratory flights, both for Robins and Pied Flycatchers. (3) Autumn releases of Robins and Pied Flycatchers under clear skies revealed a significantly larger angular dispersion in orientation for migratorily naive (first-year) birds as compared to adults. This suggests that adult birds orient with higher precision due to experience and/or that strong selection among young migrants operates to maintain orientation within narrow limits. (4) Pied Flycatchers (longdistance migrants) vanished from view significantly faster than did Robins (shortdistance migrants) when released under clear autumn skies. (5) Displacement experiments with Robins and Pied Flycatchers, captured at two different sites and then transported to a common release site, yielded inconsistent results which, in only one case out of four, could be interpreted as compensatory orientation towards the capture sites.

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

SummaryMigratory 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.

Orientation of nocturnally migrating Swainson's thrush at dawn and dusk: Importance of energetic condition and geomagnetic cues

Abstract We studied the early morning cage orientation of nocturnally migrating Swainsons Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainsons Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.

Stored fat and the migratory orientation of birds

Migration is a fundamental characteristic of the life history of many birds (Baker 1978). Birds face several different behavioural challenges during migration, including the need to deal with the high-level energetic demands of flight (Blem 1980, 1990; Pennycuick 1989; Norberg 1990). Evolution of migration is tied to the exploitation of different habitats as life-history requirements alter or environments change (Alerstam 1990; Dingle 1996). To exploit the different habitats encountered during a migratory journey requires energetic resources sufficient to reach them and the capacity to orient between different stopover sites (Dingle 1996).

Bird Orientation: Displacement Experiments with Young Autumn Migrating Wheatears, Oenanthe oenanthe, along the Arctic Coast of Russia

Migratory naive birds are thought to find the migration route from their natal site to a specific wintering area by either clock-and-compass orientation or goal area navigation. These two alternative hypotheses were tested by extensive longitudinal displacements of juvenile wheatears, hatched in northeastern Siberia, on their first autumn migration towards the wintering area in East Africa. Orientation of individual birds was repeatedly recorded in circular orientation cages, under natural clear and overcast conditions in the local geomagnetic field, during the westward displacement. During the initial part of the migration period (no stars visible), the wheatears oriented towards geographic NW-NNW under both clear and overcast skies. At the last test site, when the birds got access to directional information from the stars, their orientation changed and became bimodal with the majority of headings towards south. These results fail to support both clock-and-compass orientation and goal area navigation, and show that additional factors, not yet understood, affect the birds’ orientation in field tests.

The broken axis approach - A new way to analyse bi-directional circular data

A new technique is demonstrated that allows detection of bi-directional asymmetric modes in circular data. The method makes it possible to search for and find the best possible description of data sets that are distributed on a circle. It was primarily developed for analysis of avian orientation data, but it is equally well suited for circular data in general. Critical levels of statistically significant deviations from uniformity, according to analyses by the new technique, are provided.

Orientation cage experiments with Dunlins during autumn migration in Iceland

Migratory orientation of Dunlins Calidris alpina at sunset was studied by using orientation cages, during autumn migration in SW Iceland. The aim of the experiments was to examine whether orientation funnels can be used to record the directional behaviour of shorebirds. The results clearly demonstrate that Dunlins are capable of expressing their preferred migratory direction in funnels. Unmanipulated Dunlins showed a well-oriented mean direction towards geographical SSE which is in excellent agreement with available ringing recovery data on Dunlins linked to Iceland. This opens up the possibility to perform carefully controlled experiments to investigate the orientation and navigation mechanisms of migrating waders.