Energetic condition influences migratory decisions made by songbirds. Over-water or coastal flights are often the shortest routes but may be dangerous for landbirds because they may get caught over water where they are not able to rest and refuel. We investigated how various components of energetic condition were related to the likelihood that a migrant songbird in the Gulf of Maine region would initiate a migratory flight and the direction that it would choose. We used release tests and measurements of energy stores (fat score), plasma triglycerides, and within-day changes in body mass to investigate whether these measures of condition were related to the decisions migrants must make about when to resume migration and which direction they should go. Our results indicated that the amount of fat that a bird had at the time of release influenced the decision to initiate a flight, whereas directional decisions were influenced by both fat and within-day changes in body mass. Plasma triglyceride levels were higher in birds that initiated migratory flights; however, this does not appear to have influenced departure or directional decisions. These results indicate that migrant songbirds on stopover rely on cues about their energetic condition when making departure and orientation decisions and that the amount of fat a bird has may be the most relevant cue for these decisions.
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Vol. 128 • No. 2