Recent evidence suggests that migration may be extremely important in limiting populations of migratory songbirds. Understanding the relationship between changes in cor- ticosterone secretion and energy reserves during migration is essential to understanding how environmental conditions such as weather, food availability, predation pressure, and habitat quality may be affecting birds en route. Several studies have found that baseline corticosterone is often elevated, and response to capture and handling stress reduced (migration modulation hypothesis), in birds sampled during the migratory period. However, because neither corticos- terone secretion nor energetic condition within groups of birds sampled either in laboratory or in field varied significantly (Holberton et al. 1996, Holberton 1999), it was unknown whether corticosterone secretion could be modulated to track changes in energetic condition within the migratory period. Here, we investigated patterns of corticosterone secretion associated with variation in energetic condition in 18 free-living Hermit Thrushes (Catharus guttatus) sampled during autumn migration. Mean baseline corticosterone (mean ± SE) was 37.22 ± 7.70 ng mL−1 and increased during the 30-min interval up to 54.52 ± 6.01 ng mL−1. Lean birds were more likely to exhibit higher levels of baseline corticosterone and a reduced adrenocortical response, compared with birds that had greater energy reserves. Collectively, the results support the migration modulation hypothesis and illustrate that baseline and stress-induced corticosterone secretion can be modulated to meet the changing energy needs of birds during the migratory period.
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Vol. 121 • No. 4