Physiological costs of reproductive effort in the Wood Thrush (Hylocichla mustelina)

Parental investment during the breeding season comes at a high cost for songbirds. This is especially true for species that exhibit multi-brooding during a single season. Repeated nesting attempts caused by nest depredation, abandonment, or other causes of failure create a costly cycle of parental demand. This study evaluated corticosterone and plasma metabolite responses during each of incubation, nestling, and late season (post-breeding/moult) stages during peak nesting in a northwestern Pennsylvania (USA) population of Wood Thrush (Hylocichla mustelina). Corticosterone titers, both baseline and elevated, were found to decrease significantly throughout the breeding season. Metabolite profiles for each of beta-hydroxy-butyrate, glycerol, and triglycerides, were described for each of the breeding stages. Most notably, a strong positive relationship was observed in late season captures between corticosterone (both baseline and elevated) and cumulative number of hatched young per adult (a measure of cumulative parental effort during the breeding season). This relationship suggests that individuals that undergo high parental care demand experience a physiological shift opposite the norm for late season. Parents that exhibit high cumulative parental effort do not appear to shunt the adrenocortical response as is observed in other species, perhaps because Wood Thrush exhibit late migration initiation and molt-migration overlap as strategies to cope with high energetic demands late in the season.