Nocturnal orientation of robins, Erithacus rubecula: birds caught during migratory flight are disoriented.

Abstract

For conventional experiments on the orientation behavior of migrant birds in funnels, either hand-raised birds or birds caught during resting periods at stopover sites are generally used. Topographic circumstances at the Alpine pass Col de Bretolet at the Switzerland/France border allow the capture of birds during active migratory flight during the whole night. These birds are in full migratory disposition. We expected them to orient in the seasonally appropriate direction more accurately than birds that had not experienced migration just before the funnel experiment. Experiments with robins, however, revealed a strong influence of the moon on the orientation behavior. The birds did not orient in the seasonally expected migratory direction but showed positive phototaxis, usually toward the lightest part of the funnel in the direction opposite to the moon. When the moon was absent the robins were disoriented. Sunset experiments with robins caught during the night before the experiment revealed a strong phototactic reaction toward the setting sun. As reasons for this poor orientation in the absence of light stimuli, the influence of the topography of the mountainous region and magnetic anomalies can be excluded. It is concluded that freshly caught birds are too stressed when tested immediately after capture or that the migration direction cannot be maintained. Testing of night migrants in complete darkness is also of disadvantage.