Stress response and the value of reproduction: are birds prudent parents?

In vertebrates, stressors such as starvation or predator attacks stimulate the rapid elevation of circulating glucocorticoid hormones, triggering physiological and behavioral responses that aid immediate survival but simultaneously inhibit reproduction. This stress response has been proposed to serve as a physiological mediator of life-history trade-offs: when the value of current reproduction is high relative to the value of future reproduction and survival, a mitigated stress response is expected to enable successful breeding and maximize fitness. Using phylogenetic comparative analyses, we investigated baseline and peak stress-induced plasma corticosterone levels during parental care in 64 bird species. We found that (1) species with a higher value of the current brood relative to future breeding mounted weaker corticosterone responses during acute stress, and (2) females in species with more female-biased parental care had weaker corticosterone responses. These results support the brood value hypothesis, suggesting that the stress response evolves as an adaptive basis for life-history strategies. Further, we found that (3) baseline corticosterone correlated positively with brood value and negatively with body mass, and (4) peak corticosterone was greater in species breeding at higher latitudes. The latter findings suggest that circulating corticosterone concentrations might be matched to the anticipated demands and risks during nesting.