Developmental changes and among-sibling variation of corticosterone levelsin an altricial avian species

The postnatal development of the activity of the brain-pituitary-adrenal ax is was investigated in an altricial bird species by measurements of plasma corticosterone levels in nestling and fledgling canaries, Serinus canaria. Corticosterone was detectable (>2.6 ng/ml) in 30% of 5-day-old, 67% of 10-d ay-old, 72% of 15-day-old, and 88% of 23-day-old birds. When detectable, th e corticosterone levels of 5-day-old nestlings were comparable to the basel ine levels of adult birds. Levels were higher in 10- and 15- than in 5-day- old nestlings. The levels of 23-day-old fledglings (about 6 to 7 days after fledging) were significantly higher than those of 15-day-old nestlings. Th ey were intermediate between adult baseline and stress-induced levels. Sex did not influence this general profile, but levels varied with the order of hatching within broods. At the age of 15 and 23 days first hatched chicks had higher corticosterone levels than last hatched chicks, while second hat ched chicks had intermediate levels. These differences were not correlated with body mass. The results suggest that (1) the brain-pituitary-adrenal ax is of this altricial bird becomes fully functional after hatching and (2) b irth order within broods influences corticosterone secretion during subsequ ent stages of development. It is unlikely that the brain-pituitary-adrenal axis matures at different rates in first and later hatched chicks or that t he different levels of first and later hatched chicks were caused by captur e and handling stress. Rather, they may result from such maternal effects a s hatching asynchrony or differential concentrations of yolk steroids among the eggs in a clutch. Further studies will have to show whether this syste matic variation of corticosterone levels among siblings during early life p ersists into adulthood and how it is related to behavior and fitness. (C) 1 999 Academic Press.