Influence of Experimentally Elevated Testosterone on Nest Defence in Male Dark-Eyed Juncos

Testosterone affects the allocation of reproductive effort in male birds. Elevated testosterone causes male dark-eyed juncos, Junco hyemalis, to decrease care of dependant offspring, but this generalization is based largely on reduced provisioning rates by males treated with testosterone. Therefore, we used a predator model to explore the relationship between testosterone and nest defence, a more immediate measure of male parental effort. Control males (C-males) were more likely to respond within 10 min to a mount of an eastern chipmunk,Tamias striatus, placed at the nest than were testosterone-treated males (T-males). However, among males that did respond within 10 min, T-males initiated nest defence as fast as C-males and defended the nest with equal intensity. Females initiated nest defence more rapidly and struck the mount more often than their mates, regardless of the male’s treatment. Overall, the decreased likelihood of T-males being present for nest defence (perhaps mediated by their large home ranges) may increase predation rates at their nests and represent an important cost of elevated testosterone levels.