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

Abstract

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.