Reproductive steroids as potential mediators of parental-reproductive trade-offs in a brood parasitic species.

Avian brood parasites display enhanced annual fecundity compared with other passerine birds. Female brood parasitic brown-headed cowbirds (Molothrus ater) lay a staggering estimated 40-50 eggs year-1. We examined how reproductive steroids mediate a possible trade-off between increased annual fecundity and parental care by comparing seasonal fluctuations in steroid profiles and follicular development in cowbirds and red-winged blackbirds (Agelaius phoeniceus), a related non-parasitic species. We also used gonadotropin-releasing hormone (GnRH) administration to determine whether species variation in GnRH responsivity reflects differences in behavioral phenotypes. These correlational and experimental studies are meant to test the hypothesis that reproductive steroid profiles have diverged in these two species, possibly in such a way that mediates a reproductive-parental trade-off in cowbirds. We identified several mechanisms that could enhance annual fecundity in cowbirds, and one mechanism that would do this at the cost of parental care: elevated testosterone. These results reveal that cowbirds exhibit earlier onset of breeding as measured by follicular size and estrogen concentration. Moreover, female cowbirds produce testosterone significantly quicker and more robustly in response to GnRH administration compared with red-winged blackbirds. Species divergence in seasonal steroid profiles and responsivity to GnRH, particularly with respect to testosterone, indicate the hypothalamic-pituitary-gonadal axis exhibits consequential modifications in cowbirds that may enhance reproductive output while also possibly simultaneously inhibiting caregiving behaviors.