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 hormon e might relate to migratory condition in free-living birds, we compare d plasma levels of corticosterone in response to capture and handling stress in premigratory and migrating Gray Catbirds (Dumetella caroline nsis). Lean, premigratory catbirds exhibited stress responses typical of other nonlipogenic, nonhyperphagic bird species sampled on the bree ding or wintering grounds (i.e. low levels of corticosterone at captur e followed by significant increases over 1 h). In contrast, fat, migra tory catbirds sampled later in the season showed significantly higher baseline levels of corticosterone at capture and no significant increa se in hormone levels with handling time. We also sampled a group of mi grating Yellow-rumped Warblers (Dendroica coronata) at a stopover site . Like catbirds, migrating warblers exhibited an absence of a stress r esponse, with no significant change in corticosterone levels with capt ure and handling stress. Unlike birds sampled on the breeding grounds, there was no relationship between corticosterone levels at capture an d fat scores in premigratory and migratory catbirds, or in the migrato ry warblers. These results support out Migration-Modulation Hypothesis regarding seasonal changes in corticosterone levels, viz.: (1) an abs ence of a relationship between fat deposition and baseline corticoster one levels illustrates the permissive nature of the hormone, which rem ains elevated throughout the migratory period to facilitate hyperphagi a and lipogenesis independent of short-term changes in energetic condi tion; and (2) further elevation of corticosterone in response to acute stress is suppressed during migration to protect skeletal muscle need ed for night.