Factors influencing migratory decisions made by songbirds on spring stopover

Behavioral decisions made by migrating songbirds may depend on a variety of biotic and abiotic factors. To investigate which factors most influence songbird behavior on stopover, we related departure and directional decisions of captive birds released at the capture site to a variety of factors including, weather, date, energetic condition, age, sex, and species. We captured spring migrants during the day, released them after sunset, and visually assessed whether they departed the study site and if so, in which direction. Departure was strongly influenced by wind direction and energetic condition, especially fat stores. The proportion of birds departing increased as the season progressed. Directional decisions were also strongly influenced by energetic condition, particularly fat stores and plasma triglyceride levels. Wind direction also influenced the direction in which birds made migratory flights. While energetic condition, stage of migration, and weather seem to be important proximate determinants in departure and directional decisions, habitat availability and other factors may also need to be considered.