In response to high densities of red-winged blackbird (Agelaius phoeniceus) in the northern Great Plains, natural resource managers have proposed population management plans such as reducing breeding adult numbers. However, data on compensatory effects of variation in nesting densities on the reproductive performance of red-winged blackbirds that could counteract effects of breeding population reductions is lacking. Maternally derived hormones in egg yolks could act as a compensatory mechanism by which adult social interactions affect offspring success. We conducted simulated territorial intrusions by placing caged blackbird females near nesting female red-winged blackbirds to quantify effects of increased density and associated social interactions on behavior, yolk hormones, and reproductive output. We measured aggressive behavior, maternally derived testosterone and corticosterone present in the yolks of eggs and other measures of reproductive performance. Aggressive behavior was greatest in females receiving simulated territorial intrusions compared to females not receiving simulated intrusions but otherwise we observed few differences in reproduction among the experimental groups. Maternally derived testosterone and corticosterone concentrations in eggs were not significantly different among experimental groups. However, brown-headed cowbird (Molothrus ater) parasitism was greater in, and survival probability was lower for, nests of females receiving simulated territorial intrusions. We conclude that maternally-derived testosterone and corticosterone in eggs resulting from increased social interactions among nesting females are not likely density dependent regulatory mechanisms for red-winged blackbird populations.
Bush et al. (Fri,) studied this question.