Modulation of the adrenocortical stress response in neotropical migrants during autumn migration

Seasonal changes in corticosterone metabolism have been implicated in the migratory physiology of passerines. To investigate how this hormone might relate to migratory condition in free-living birds, we compared plasma levels of corticosterone in response to capture and handling stress in premigratory and migrating Gray Catbirds (Dumetella carolinensis). Lean, premigratory catbirds exhibited stress responses typical of other nonlipogenic, nonhyperphagic bird species sampled on the breeding or wintering grounds (i.e. low levels of corticosterone at capture followed by significant increases over 1 h). In contrast, fat, migratory catbirds sampled later in the season showed significantly higher baseline levels of corticosterone at capture and no significant increase in hormone levels with handling time. We also sampled a group of migrating Yellow-rumped Warblers (Dendroica coronata) at a stopover site. Like catbirds, migrating warblers exhibited an absence of a stress response, with no significant change in corticosterone levels with capture and handling stress. Unlike birds sampled on the breeding grounds, there was no relationship between corticosterone levels at capture and fat scores in premigratory and migratory catbirds, or in the migratory warblers. These results support out Migration-Modulation Hypothesis regarding seasonal changes in corticosterone levels, viz.: (1) an absence of a relationship between fat deposition and baseline corticosterone levels illustrates the permissive nature of the hormone, which remains elevated throughout the migratory period to facilitate hyperphagia and lipogenesis independent of short-term changes in energetic condition; and (2) further elevation of corticosterone in response to acute stress is suppressed during migration to protect skeletal muscle needed for night.