An endocrine basis for trade-offs between immediate survival and reproductive success in arctic- and temperate -breeding yellow warblers, Dendroica petechia

Variation in physiological and/or behavioral responses that help maximize individual reproductive success may reflect ways different environmental factors affect time and energy demand. For birds, Arctic- and temperate-breeding conspecifics may employ adaptations that reflect trade-offs between immediate survival and reproductive success. Compared to their temperate counterparts, Arctic breeders face short breeding seasons, unpredictable environments, and may increase their threshold for immediate life-saving responses that normally come at the expense of reproductive success. To test this hypothesis, I investigated the adrenocortical response to stress of Yellow Warblers at both ends of their north-south breeding distribution during breeding stages representing notable differences in parental investment. Arctic males showed a significantly reduced stress response during nestling provisioning (when their parental contribution was highest), while temperate males showed no such reduction. Females did not differ in their stress response at either site during incubation or provisioning. They exhibited a lower response than did temperate males during both stages, and a response similar to that of provisioning Arctic males. Provisioning rate, a measure of parental expenditure, did not differ as a result of sex or latitude. In a field experiment, Arctic males manipulated to experience nestlings earlier than predicted showed no reduced response, and males manipulated to experience nestlings later than predicted showed a reduced response even when the eggs had not yet hatched, indicating that nestlings were not the cue involved in the reduction. An endogenous clock and/or prolactin may be involved with the reduction. The level at which the hypothalamic-pituitary-adrenal axis may be modulated remains unclear, but data from a laboratory study indicate that there are likely to be direct costs, to the individual, of not exhibiting a stress response when needed for immediate survival. Overall, these data show that a reduced stress response during breeding occurs when the risk for brood loss is greatest and when the degree of parental investment is highest. While the exact mechanisms involved with the reduction remain unknown, these results add to our understanding of the physiological and behavioral mechanisms associated with reproductive success in birds, and how different populations may cope with constraints on time and energy.