Postnuptial molt of the song sparrow on Mandarte Island in relation to breeding

Abstract

In the summer of 1978 we made 133 captures of 69 adult song sparrows on Mandarte Island, B.C., where the detailed breeding history of all birds was known. Molt condition and weight of all trapped individuals was noted. Molt follows the usual pattern for passerines, but secondary molt does not finish until a few days after primary molt. In the population, molt lasts from June to September, and individual molt takes about 50 days. Both sexes begin molt simultaneously. Birds that stop breeding early molt at the same time as nonbreeding individuals, but late-breeding song sparrows postpone molt. Birds that stop breeding early start molt just before the young become independent. In late breeders, feeding fledglings and molt overlap, with overlap increasing the later breeding ends. Old birds molt later because they tend to breed later and because young birds begin molt sooner after their last young hatch.Body weight increases during molt and decreases after molt, but remains about 1 g heavier than premolt weight. Birds breeding later began molt lighter.We suggest that the molt period on Mandarte is earlier than in other populations because breeding begins earlier and because individuals must finish molting before autumn territorial behaviour begins.