Effects of simulated territorial intrusions on female behavior and maternally-derived hormones present in the eggs of red-winged blackbird

RACHEL M. BUSH; SUSANA PELUC; GEORGE M. LINZ; WILLIAM J. BLEIER; WENDY L. REED; MARK E. CLARK

Congreso; Reunión Argentina de Ornitología; 2017

In the northern Great Plains of US high nesting densities of red-winged blackbird (Agelaius phoeniceus) have prompted land managers and wildlife agencies to develop population management plans. However, no empirical data are available to examine the effects of variation in nesting densities on the reproductive performance of red-winged blackbirds. Maternally-derived hormones present in egg yolks offer a potential compensatory mechanism by which adult social interactions can affect offspring success. We conducted simulated territorial intrusions using caged conspecific females on nesting female red-winged blackbirds to examine effects of increased density and associated social interactions on aggressive behavior, maternally-derived yolk hormones, and reproductive performance. Aggressive behavior was greatest in Treatment females (i.e., those which received simulated territorial intrusions using a caged conspecific) compared to Control (i.e., those which received presentations of empty cages) and No-cage Control females (i.e., those which received no presentations), but few differences in reproductive output resulted among the experimental groups. Maternally-derived testosterone and corticosterone concentrations measured in eggs did not vary among the experimental groups. However, rates of parasitism by brown-headed cowbirds (Molothrus ater) were higher, and daily nest survival probability was lower for Treatment females compared to the Control and No-cage Control females. Our results suggest that the density-dependent factors influencing yolk testosterone and corticosterone concentrations interact synergistically rather than through a simple path of increased social interactions resulting in increased concentrations in eggs in the red-winged blackbird.