Morph-Specific Variation in Baseline Corticosterone and the Adrenocortical Response in Breeding White-Throated Sparrows (Zonotrichia albicollis)

Abstract

The adrenocortical response to perturbation varies considerably within and across avian species during breeding. Among the factors hypothesized to influence this variation is the degree of parental care expressed by an individual. To increase our understanding of the relationship between parental care and the adrenocortical response, we examined corticosterone secretion in the polymorphic White-throated Sparrow (Zonotrichia albicollis) during three breeding stages: prelaying, incubation, and nestling. In addition, we reevaluated previously described morph differences in parental behavior. Tan-striped males provisioned nestlings at higher rates than white-striped males, which is consistent with earlier findings. However, in contrast to previous studies, provisioning rates were similar between female morphs and higher than those exhibited by either male morph. We found that, in males, the more parental morph (tan-striped) expressed a reduced adrenocortical response during the nestling stage, but the less parental morph (white-striped) did not. Females of both morphs exhibited similar and reduced adrenocortical responses during parental stages. In addition, we observed morph differences in baseline corticosterone during the nestling stage: less parental birds (white-striped males) had higher baseline corticosterone levels than more parental birds (females and tan-striped males). These findings support the hypothesis that modulation of baseline corticosterone and the adrenocortical response during breeding is a function of the degree of parental care expressed by an individual. By influencing how parents balance self-maintenance and parental care on a daily basis, or in the face of perturbation, baseline corticosterone and the adrenocortical response may play a role in the alternative reproductive strategies of White-throated Sparrow morphs.