Rl. Holberton et al., MODULATION OF THE ADRENOCORTICAL STRESS-RESPONSE IN NEOTROPICAL MIGRANTS DURING AUTUMN MIGRATION, The Auk, 113(3), 1996, pp. 558-564
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.