How seasonality in the southern hemisphere affects migration of austral migrant Tyrannus savana

We lack a thorough understanding of the degree to which the seasonal changes in locations of animals are driven by the timing and sequence of resource availability across the annual cycle, a necessary component for their conservation in a changing global climate. However, recent advancements in animal tracking technology offer an unprecedented ability to explore how individual animals track seasonal changes. This has been an especially fast-growing question in the study of bird migrations with evidence supporting a major role of seasonality in temperature, rainfall and primary productivity in driving seasonal movements of individuals. To test which environmental factors (temperature, rainfall and/or primary productivity) affect seasonal distributions of austral migrants, we used Maxent models of presence-only data gathered from 56 Tyrannus savana individuals fit with light-level geolocators at five breeding locations in Bolivia, Brazil, and Argentina. We found that T. savana tracked predominantly with temperature and NDVI (a measure of primary productivity) during the breeding months in southern South America, but tracked with temperature and rainfall during both spring and fall migrations, and primarily with rainfall while overwintering in northern South America. This supports the general idea that seasonality is important to annual routines in migratory birds, even within South America where seasonality is driven largely by wet-dry cycles. Since wet-dry seasons vary inter-annually, migration in the austral system may be more flexible in timing than in boreal systems where seasonal movements are strongly linked to temperature.