Winter Versus Summer Explanations of Delayed Plumage Maturation in Temperate Passerine Birds

The subadult breeding plumages of yearling males of temperate passerine birds have generally been assumed to be an adaptation to their first breeding season. We challenge this assumption with the alternative that subadult plumages are an adaptation to the first winter and are retained into the first breeding season because of the high cost of spring molts. To test these alternatives, we examined the winter appearance and the status of the spring molt in 105 North American passerines that are sexually dichromatic in the winter and in the summer. Two observations could have provided strong support for subadult plumages' being a summer adaptation. First, in 42 species with an extensive spring molt, young males could either remain entirely female-like or become more female-like in the summer than in the winter. No species does this. Second, young males whose first breeding plumage is either perfectly female-like or intermediate between adult males and females could acquire the female-like aspect of their spring appearance by growing female-like feathers in the spring molt. Again, no species is known to do this. Several lines of evidence support the assumption of winter adaptation. First, every species in which the first-breeding plumage of males is subadult also features a first-winter plumage that is subadult. The continued presence of a subadult plumage into the breeding season always is a consequence of either no spring molt or an incomplete spring molt. Second, as far as we currently know, spring molts that are incomplete always make young males more like adults because all of the feathers grown resemble those of the adult-male breeding plumage. Finally, a number of species have complete molts of their body feathers in the spring; in all these species, young males lose their subadult winter plumage and become like adult males for their first breeding season. We subdivide the earlier winter-adaptation hypothesis into two possible adaptive explanations of the significance of winter subadult plumages and a hypothesis of molt constraints. Only the molt-constraints hypothesis competes with earlier hypotheses proposed to explain subadult breeding plumages. Unlike these earlier ideas, the molt-constraints hypothesis argues that subadult plumages are maladaptive in the breeding season but retained into that season because a complete spring molt is impossible.