Swainson’s Thrushes show little response to winds when negotiating the Gulf of Mexico

Migrating birds should benefit from flying with wind support, as it can minimize the energetic costs of flight and increase migration speed. However, to what extent an individual benefits from optimizing wind support depends on its flight efficiency, variation in available winds, and the costs of seeking most favorable winds. To examine how Swainson’s thrushes responded to winds when negotiating the Gulf of Mexico, we combined long-term banding data and automated radio-telemetry data with modeled wind fields to examine stopover behavior and generate individual based models of trans-gulf flight. Thrushes stopped over more frequently in years with less supportive winds. Even though the nights they chose to stop over and depart over the Gulf were more supportive than expected, they often chose nights that were unsupportive, and only avoided the least favorable nights (wind profits < -5 m/s). Moreover, birds did not appear to select altitudes or routes with the most supportive winds when crossing the Gulf, perhaps because they can tolerate large variation in winds, or because, in this system, finding and using the most supportive winds has more energetic costs (climbing, maneuvering in increased turbulence) than benefits (fewer travel miles, lower airspeeds). The finding that Swainson’s thrushes with sufficient fat stores are able to cross the Gulf successfully even in unsupportive winds challenges the notion that the Gulf should be considered an ecological barrier for migratory birds, and identifies a need to redefine the term to reflect true, context-dependent impediments to movement.