  Organisms are frequently exposed to challenges during development, such as
poor weather and food shortage. Such challenges can initiate the hormonal
stress response, which involves secretion of glucocorticoids. Although the
hormonal stress response helps organisms deal with challenges, long-term
exposure to high levels of glucocorticoids can have morphological, behavioral,
and physiological consequences, especially during development. Glucocorticoids
are also associated with reduced survival and telomere shortening. To
investigate whether brief, acute exposures to glucocorticoids can also produce
these phenotypic effects in free-living birds, we exposed wild tree swallow
(Tachycineta bicolor) nestlings to a brief exogenous dose of cort once per day
for five days and then measured their morphology, baseline and stress-induced
corticosterone levels, and telomere length. We also deployed radio tags on a
subset of nestlings, which allowed us to determine the age at which tagged
nestlings left the nest (fledged) and their pattern of presence and absence at
the natal site during the post-breeding period. Corticosterone-treated
nestlings had lower mass, higher baseline and stress-induced corticosterone,
and reduced telomeres; other metrics of morphology were affected weakly or not
at all. Our treatment resulted in no significant effect on survival to
fledging, fledge age, or age at first departure from the natal site, and we
found no negative effect of corticosterone on inter-annual return rate. These
results show that brief acute corticosterone exposure during development can
have measurable effects on phenotype in free-living tree swallows.
Corticosterone may therefore mediate correlations between rearing environment
and phenotype in developing organisms, even in the absence of prolonged
stressors.
