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

Abstract

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 spri...