INFLUENCE OF EXPERIMENTALLY ELEVATED TESTOSTERONE ON NEST DEFENSE IN DARK-EYED JUNCOS

Testosterone affects the allocation of reproductive effort in male bir ds. 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 test osterone. Therefore, we used a predator model to explore the relations hip between testosterone and nest defence, a more immediate measure of male parental effort. Control males (C-males) were more likely to res pond within 10 min to a mount of an eastern chipmunk, Tamias striatus, placed at the nest than were testosterone-treated males (T-males). Ho wever, among males that did respond within 10 min, T-males initiated n est defence as fast as C-males and defended the nest with equal intens ity. Females initiated nest defence more rapidly and struck the mount more often than their mates, regardless of the male's treatment. Overa ll, the decreased likelihood of T-males being present for nest defence (perhaps mediated by their large home ranges) may increase predation r ates at their nests and represent an important cost of elevated testos terone levels. (C) 1998 The Association for the Study of Animal Behavi our.