Examining the potential decoupling of reproductive-stress interactions in blackbirds with stark differences in reproductive strategy.

Stress and reproduction are often linked through interactions between the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes such that stress can suppress reproduction. It is unclear, however, whether species with enhanced annual fecundity exhibit modifications in this interaction. We addressed this question using a comparative study of two closely related blackbird species that differ markedly in parental care and reproductive output: the brown-headed cowbird (Molothrus ater), an obligate brood parasite that has enhanced annual fecundity without parental care, and the red-winged blackbird (Agelaius phoeniceus), which does not have enhanced annual fecundity and provides parental care. We quantified restraint-induced corticosterone and circulating testosterone, 17β-estradiol, and ovarian follicle size in breeding adult females. We also characterized testosterone and corticosterone profiles in nestlings and adults of both species. Corticosterone responses to restraint did not differ between species or age classes but age-related trajectories diverged as corticosterone declined from nestlings to adults in cowbirds but increased in red-winged blackbirds. On the other hand, testosterone concentrations remained stable across species and ages. In adult red-winged blackbirds, reproductive state positively predicted corticosterone responses to restraint, such that individuals with higher testosterone, estradiol, or larger follicles mounted stronger corticosterone responses. In contrast, cowbirds showed no relationship between reproductive state and stress-induced corticosterone. These findings indicate that cowbirds exhibit an altered relationship between reproductive and stress physiology that suggests a functional decoupling between HPA and HPG activity. Such decoupling may support enhanced fecundity in obligate brood parasites.