A test of the migration-modulation hypothesis in a non-passerine Neotropical migrant, the Blue-winged Teal Anas discors

Blue-winged Teal ( Anas discors ) initiate southward migration from their North American breeding grounds in late summer, making trans-Gulf flights to overwinter in South America. As such, this species displays a fall migratory pattern unique among North American waterfowl and similar to that of many New World passerines that migrate to and overwinter in the Neotropics. Corticosterone, the major avian hormone of stress and energy regulation, is known to influence migratory physiology in passerines. The migration-modulation hypothesis (MMH), a pattern of corticosterone secretion characterized by an elevated adrenocortical baseline and a reduced adrenocortical response to a standardized stressor, has been documented in several passerine and shorebird species during migration. However, little is known about the adrenocortical response in migratory waterfowl. We sampled Blue-winged Teal during fall ( n = 8) and spring migrations ( n = 10), and compared their adrenocortical profiles to those of breeding Teal ( n = 20). Baseline corticosterone did not differ among groups; however, migrating Teal in both fall and spring had a significantly reduced adrenocortical response. This reduction in the acute adrenocortical response during migration supports the MMH, and suggests the existence of a mechanism similar to that of Neotropical migratory passerines, which is hypothesized to protect flight muscle from the catabolic effects of elevated corticosterone.