LOVE, O.P.; BIRD, D.M.; SHUTT, L.; McGill University, Montreal; McGill University, Montreal; Canadian Wildlife Service, Hull: Plasma corticosterone in captive sibling American kestrels: effects of age, hatching order and hatching asynchrony

Sibling variation in adrenocortical function was investigated in postnatally developing, asynchronously-hatching American kestrels (Falco sparverius) by measurements of baseline and acute stress-induced levels of corticosterone at ages 10, 16, 22 and 28 days post-hatching. There was a significant effect of hatching order on both baseline and stress-induced acute response corticosterone levels through development and these effects grew stronger through development. First-hatched chicks exhibitted higher baseline levels than later-hatched chcisk throughout development and higher stress-induced levels during the latter half of development. Furthermore, there was significant hatching span x hatching order interaction on both baseline and acute stress-induced corticosterone levels during development. Hatching span was positively correlated with both measures of corticosterone and body mass in first-hatched chicks, but was negatively correlated through most of development in last-hatched chicks. It is known that hatching asynchrony creates mass and size hierarchies within kestrel broods and we suggest that differences in adrenocortical function among siblings may be one physiological mechanism by which these competitive hiearchies are maintained.