Variation in plasma corticosterone in migratory songbirds: a test of the migration-modulation hypothesis.

Physiological mechanisms underlying migration remain poorly understood, but recent attention has focused on the role of the glucocorticoid hormone corticosterone (CORT) as a key endocrine regulator of migration. The migration-modulation hypothesis (MMH) proposes that baseline plasma CORT levels are elevated in migratory birds to facilitate hyperphagia and lipogenesis and that further elevation of CORT in response to acute stress is suppressed. Consequently, CORT may be a poor indicator of individual condition or environmental variation in migratory birds. We tested the MMH by measuring baseline and stress-induced CORT in common yellowthroats (Geothlypis trichas) during fall migration over 2 consecutive years in the Revelstoke Reach drawdown zone, a migratory stopover site affected by local hydroelectric operations. Birds had low baseline CORT at initial capture (<5 ng/mL) and then showed a robust stress response, with CORT increasing to ca. 50 ng/mL within 10-20 min. Our data therefore do not support the MMH. Baseline CORT did not vary with body mass, time of capture, Julian day, or year, suggesting that variable flooding regimes did not affect baseline CORT. Individual variation in the rate of increase in CORT was correlated with Julian day, being higher later in the migration period. Our data suggest that plasma CORT can be a useful metric in migration studies.