A Description and Exploration of the Potential Causes of Intra-Tropical Migration in the Neotropical Bird “Purple Martin” (Progne Subis)

Abstract

This study explores the potential causes of intra-tropical migration by neotropical songbirds, some of whom undertake extensive migrations greater than 100 km within their wintering rage in the Amazon. Using geolocator-retrieved data on Purple Martin (Progne subis) migration, the spatial and temporal aspects of the movements are described. After arrival at their initial winter roost in Northern Brazil, sixty percent migrated to a second site in the eastern Amazon, travelling a mean distance of 813 km. It was predicted that weather or insecticide-induced reductions in food availability drives this movement, mediated by individual factors such as sex, age, fall migration timing, and distance. Statistical analyses reveal a general movement toward agricultural landscapes, but no significant differences in temperature or rainfall patterns between initial and subsequent roost sites. Older, experienced birds and those arriving at their first roost sites later are more likely to be intra-tropical migrants than younger birds or early arrivals.