A TEST OF AN ASSUMPTION OF DELAYED PLUMAGE MATURATION HYPOTHESES USING FEMALE TREE SWALLOWS

The Tree Swallow (Tachycineta bicolor) is one of only two North Americ an birds in which females, and not males, have delayed plumage maturat ion. The hypothesis that plumage differences are the result of differe nces in competitive abilities among individuals is tested by comparing brown and blue Tree Swallow females in terms of foraging abilities, a s determined by the growth of nestlings under their care without the h elp of the males, and in terms of body size and nutritional condition. Blue and brown females were not different when comparing the growth a nd fledging success of nestlings under their care. Blue females were l arger and, even after accounting for body size differences, heavier an d in better body condition than brown females. While brown plumage ser ves to reduce male aggression, blue plumage probably serves as a signa l of dominant status in intrasexual confrontations. This signalling sy stem is not open to cheating because of the larger size and better bod y condition of blue females.