Energy Demands of Migration on Red-Eyed Vireos, \u3ci\u3eVireo olivaceus\u3c/i\u3e

We studied the behavior of red-eyed vireos (Vireo olivaceus)—including their length of stopover, rate of mass change (lipid deposition), and foraging behavior—in relation to energy demand after their spring migration across the Gulf of Mexico. We labeled each 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 during their stopover than birds that arrived with unmobilized fat reserves. Several aspects of the birds\u27 foraging behavior varied in relation to mean body mass on observation days: Lean migrants (1) broadened their use of microhabitat but not their vertical distribution of foraging locations, (2) expanded their feeding repertoire, (3) moved at a higher mean velocity while averaging the same frequency of foraging movements, and (4) increased their degree of turning after a feeding attempt. We suggest that the gain of mass by fat-depleted birds is a compensatory response to energy demand mediated by changes in foraging behavior