Long-distance bird migrants adjust their foraging behavior in relation to energy stores

Abstract

Migratory birds can anticipate heightened energy demand and increased uncertainty that energy demands will be satisfied during their annual migration phase. Optimal migration theory predicted that stopover decisions could be affected by energetic condition, foraging opportunity, and timing of migration. Using the stopover migratory thrushes following spring passage across the Gulf of Mexico, we examined the interactions of foraging behavior, energetic condition, and rate of fat deposit. Birds were mist-netted, weighted, estimated for fat storage, and released after being banded with a unique numbered aluminum leg band. For behavioral observation, birds were separated to two groups: “lean day birds” when daily average body mass of captured birds was near or below fat free mass and “fat day birds” when daily average body mass of captured birds was above fat free mass. A subsample of birds with known fat condition was banded with unique combination of color-band(s) for re-sight and confirmation of relationship between energetic condition and foraging behavior. Nearly 50% of the migrants had no visible fat left upon crossing the Gulf of Mexico and were near fat-free mass. Lean birds expanded their foraging repertoire, broadened their use of substrate, and foraged at a faster pace than did birds with fat stores remaining following arrival. Lean migrants tended to gain more mass and gain mass at a faster rate. Our data suggest that the energetic condition upon arrival at a stopover site affects foraging behavior and physiological conditions upon departure, which may further affect stopover length and the decisions of stopover en route.