Subadult plumage in the house finch and tests of models for the evolution of delayed plumage maturation

-In many passerine species males do not attain definitive breeding plumage until after their first potential breeding season. Such delayed plumage maturation has been the subject of numerous studies, most of which have focused on the adaptive value of femalelike plumage for young males in single populations. Thus, although studies of delayed plumage maturation have been presented as tests of evolutionary hypotheses, most have lacked an appropriate comparative perspective. Moreover, most studies of subadult plumage in passerines have been conducted without information on the general function of ornamental plumage in the species being studied. A study of delayed plumage maturation in the House Finch (Carpodacus mexicanus) provided an opportunity to overcome these problems. The function and proximate control of ornamental plumage coloration in House Finches have been studied extensively. In addition, House Finches occupy a wide range across North America with approximately 15 subspecies recognized, and populations vary in expression of delayed plumage maturation. In most populations, including the most familiar subspecies, C. m. frontalis, males acquire definitive basic plumage in their first prebasic molt. In at least three populations, C. m. griscomi of southern Mexico, C. m. amplus of Guadalupe Island, and Great Basin populations of C. m. frontalis, however, males do not acquire definitive plumage until the completion of their second prebasic molt, spending their first potential breeding season in a female-like plumage. Using a composite phylogeny and parsimony analysis, I show that delayed plumage maturation likely was lost in the ancestral House Finch population and was regained independently by the three taxa in which it is now observed. Thus, delayed plumage maturation in the House Finch is a derived trait. In addition, I show that the acquisition of delayed plumage maturation is associated with a reduction in the extent of ornamental plumage (i.e. a reduction in patch size). These observations support the idea that delayed plumage maturation in the House Finch is an adaptation to increased production costs of red ornamental display plumage in some populations. Received 28 November 1995, accepted 25 April 1996. DELAYED PLUMAGE MATURATION describes a developmental pattern generally limited to males, whereby individuals do not acquire definitive breeding plumage until after their first potential breeding season (Rohwer et al. 1980, Rohwer and Butcher 1988). Studies of delayed plumage maturation most frequently concern Temperate Zone passerines that spend only one potential breeding season in a subadult plumage (Rohwer and Butcher 1988, Butcher and Rohwer 1989, Zack and Stutchbury 1992). Despite a substantial literature on delayed plumage maturation (see Zack and Stutchbury 1992), our understanding has advanced relatively little since interest in the topic was first ignited by Rohwer et al. (1980). I believe that the ma1 E-mail: geoffrey.hill@ag.auburn.edu jority of research on delayed plumage maturation in passerines suffers from two fundamental problems: (1) lack of a general understanding of the function of ornamental plumage coloration, and (2) lack of an appropriate comparative approach to testing hypotheses for the evolution of delayed plumage maturation. The first of these problems clearly is a case of putting the cart before the horse. Ornamental plumage coloration, either bright structural coloration, bright carotenoid-based coloration, or highly contrasting melanin-based coloration, is universally assumed to function as a signal in avian communication (Butcher and Rohwer 1989). Signaling generally is believed to occur in the context of either intersexual or intrasexual interactions (Butcher and Rohwer 1989), but predator-prey interactions also have been considered (Gotmark 1992, 1993). An assump-