WILSON, C.M.*; HOLBERTON, R.L.: The Endocrine Basis for Trade-offs between Immediate Survival and Reproductive Success in Arctic- and Temperate-breeding Yellow Warblers

While temperate breeding birds can renest or produce more than one successful clutch each year, Arctic-breeders are unlikely to raise more than one clutch annually due to shorter breeding seasons and unpredictable weather. Despite these constraints, many birds successfully breed in the Arctic, suggesting that trade-offs between individual risk and reproductive success differ between Arctic- and temperate breeders. The mobilization of higher plasma levels of corticosterone helps individuals survive environmental perturbations, particularly for those living in extreme and stochastic environments. However, high concentrations of corticosterone are incompatible with reproduction. We have asked how high latitude-breeding birds with limited opportunities to breed cope with the trade-off between the personal benefits of the corticosterone stress response needed in such an environment, and the possible cost in reproduction that such a response may incur. In 1999 and 2000, we found that Arctic-breeding male Yellow Warblers, who must contribute significantly to parental care, reduce acute corticosterone secretion during times of highest parental expenditure. The cost of this response to the individual and the mechanism by which it occurs are under investigation. Other temperate breeding species, to date, have not shown a similar reduction; samples from temperate-breeding Yellow Warblers are currently being analyzed. If temperate breeding males do not exhibit a similarly reduced stress response, this phenomenon may represent a mechanism by which two different strategies expressed within the same species might maximize reproductive success under disparate ecological constraints.