Energy demands of migration on red-eyed vireos, Vireo olivaceus

We studied the behavior of red-eyed vireos (Vireo olivaceus)—includingtheir length of stopover, rate of mass change (lipid deposition),and foraging behavior—in relation to energy demand aftertheir spring migration across the Gulf of Mexico. We labeledeach foraging bird as ‘lean’ or ‘fat’based on the average condition of birds captured on that day.Lean birds stayed longer and were more likely to gain mass duringtheir stopover than birds that arrived with unmobilized fatreserves. Several aspects of the birds' foraging behavior variedin relation to mean body mass on observation days: Lean migrants(1) broadened their use of microhabitat but not their verticaldistribution of foraging locations, (2) expanded their feedingrepertoire, (3) moved at a higher mean velocity while averagingthe same frequency of foraging movements, and (4) increasedtheir degree of turning after a feeding attempt. We suggestthat the gain of mass by fat-depleted birds is a compensatoryresponse to energy demand mediated by changes in foraging behavior.