Relation Between Migratory Activity and Energetic Condition Among Thrushes (Turdinae) Following Passage Across the Gulf-of-Mexico
Whereas endogenous time programs are thought to control different aspects of bird migration, regulation of migratory behavior is sensitive to acute ecological factors. The relation between migratory activity and energetic status was studied by monitoring day (presumptive feeding) and night (migratory) locomotor activity of captive North American thrushes (Hylocichla mustelina, Catharus fuscescens, C. ustulatus, and C. minimus) immediately after spring passage across the Gulf of Mexico. (1) Lean migrants, regardless of species, displayed less night activity than migrants with undepleted fat stores. (2) Lean birds were more active during the day (presumptive feeding behavior) than fat birds, regardless of species. (3) Nocturnal restlessness resumed as lean individuals replenished fat stores, whereas day activity decreased as lean birds regained mass. (4) Catharus thrushes, which travel longer distances than Wood Thrushes before reaching breeding grounds, displayed more activity on a per-night basis. Our results support the idea that a migrant's energetic status modifies the programmed coursed of migration.