Singing in Africa: no evidence for a long supposed function of winter song in a migratory songbird

Animal sexual signals have been well studied in the context of mate attraction, but less attention has focused on the function of signals produced at other times of the year. Despite decades of research on bird song, we still do not know why many migratory songbirds also sing intensely on their wintering grounds. It has long been assumed that winter song is used to defend individual feeding territories; however, this assumption has never been tested. I studied willow warblers (Phylloscopus trochilus) that regularly sing on their African wintering grounds to test this supposition using a combination of behavioral experiments and radio telemetry. Contrary to expectations for individual territory defense, I found that individuals foraged in conspecific groups with fluid group membership, moved itinerantly from site to site rather than remaining in stable territories, and were highly aggressive toward model presentations combined with song playback, but in groups rather than individually. These results suggest a puzzling alternative function for winter song—singing for group territory defense—where territories are short term and group membership is not fixed. This work highlights the scarcity of our knowledge on the nonbreeding behavior of Palearctic–African songbirds and the unexpected complexity of avian social organization during this period of the annual cycle.