Testosterone and Avian Life Histories: Effects of Experimentally Elevated Testosterone on Prebasic Molt and Survival in Male Dark-Eyed Juncos

Abstract

Male Dark-eyed Juncos (Junco hyemalis) that breed in Virginia begin their prebasic molt after breeding has ended, usually in August. Almost all males caught in late October have completed the molt. In 1989, we obtained anecdotal evidence that males whose testosterone (T) we maintained at artificially elevated levels beyond the end of the breeding season postponed or suppressed prebasic molt. To test the effect of T experimentally, in spring 1990 we implanted some males (T-males) with testosterone and others (Cmales) with empty implants, and we released both groups to breed. We caught some members of both treatment groups in October and removed their implants. The T-males had delayed their prebasic molt, while the C-males were molting on schedule. Other implanted Tand C-males were not caught; these carried their implants into winter. Next spring we examined surviving males whose implants we had removed in October as well as males whose Tand C-implants had not been removed. T-males whose implants we had removed had molted completely, despite their delayed start, whereas T-males whose implants we had not removed had not molted. Still-implanted C-males had molted. We compared the minimum overwinter survival (i.e., return rates in spring) of the treatment groups. Tand C-males whose implants we had removed in October returned at the same rate, but among males whose implants we had not removed, significantly fewer T-males than C-males returned. The transition between reproduction and molt of male juncos apparently can be blocked by preventing the normal seasonal decline in T. This suggests a physiological basis for a possible trade-off between time allocated to reproduction and time allocated to molt. Our results indicate that males could maintain high T and prolong breeding, possibly into October, and still molt completely with no adverse effects. We consider why such a modification of schedule has not occurred. However, postponement of molt beyond some date in autumn, possibly late October, suppresses it altogether, as indicated by the failure to molt of the returning T-males whose implants we did not remove. This treatment group apparently suffered higher overwinter mortality, and we consider possible reasons.