A previously unknown plumage of first-year indigo buntings and theories of delayed plumage maturation

-First-year, but not adult, Indigo Buntings (Passerina cyanea) have a previously unknown supplemental plumage. The presupplemental molt includes all of the rectrices, the outermost but not the innermost primaries, and, typically, the three innermost secondaries and all body feathers. In this molt, young females exchange one adult-femalelike plumage for another, while young males exchange an adult-femalelike plumage for one that matches that of adult males in winter. Thus, in their first year Indigo Buntings wear: first, the juvenile plumage, the body feathers of which begin replacement before the tail is fully grown; second, the first basic plumage, which in both sexes is entirely femalelike in coloration and includes the juvenile remiges and rectrices; third, the supplemental plumage, assumed either prior to fall migration ( 90% of individuals) and in which obvious sexual dichromatism is first achieved; and fourth, the first alternate plumage, acquired in a prolonged and often incomplete prealternate molt of body feathers that occurs during February, March, and April on the wintering ground and during the spring in the United States. Because almost all of the femalelike first basic plumage of young males is lost in the presupplemental molt, this plumage almost certainly is an adaptation to conditions encountered either in the fall or early in the first winter. Furthermore, the ensuing supplemental plumage cannot be compromised by color requirements of the first breeding season because of the intervening prealternate molt; thus, the adult-malelike plumage produced by the presupplemental molt likely evolved to meet a change in signaling requirements that occurs in early winter. The signaling function of this plumage is unknown. Because this supplemental plumage of young males resembles the winter plumage of adult males and because all feathers grown by young males in their first prealternate molt resemble those of the adult male breeding plumage, the female mimicry hypothesis of Rohwer et al. (1980) is untenable for the subadult breeding plumage of yearling male Indigo Buntings. Received 16 August 1984, accepted 5 October 1985. THEORIES of delayed plumage maturation for male passerines may be divided into sets of summer and winter adaptation hypotheses (Rohwer et al. 1983, Rohwer and Butcher unpubl.). Two hypotheses consider subadult plumages to be an adaptation to conditions of the first potential breeding season. The cryptic hypothesis (Lack 1954, Selander 1965) considers both the infrequency of breeding by subadults and their subdued appearance to be adaptive responses to the inferiority of subadults in competition with older males. The female mimicry hypothesis (Rohwer et al. 1980, Rohwer 1983) views failures by subadults to breed as maladaptive, and their femalelike plumage as an adaptation by which young males increase, through deception, the likelihood of acquiring female-worthy territories and of breeding in their first summer. Two other hypotheses consider subadult plumages as an adaption to conditions encountered in the first winter. The status signaling hypothesis (Rohwer 1975, Ketterson 1979, Rohwer and Ewald 1981) treats the subdued first winter plumage of young birds as a badge of subordinance that reduces aggression from adults and, thus, facilitates flocking with adults. The cryptic hypothesis (Ewald and Rohwer 1980) argues that young birds are inconspicuous in winter to escape detection by predators or by dominant adult birds. If a subdued first winter plumage is more valuable than a conspicuous first breeding plumage, and if spring molting is constrained, then subadult breeding plumages could be maladaptive. Both summer hypotheses presume subadult breeding plumages are adaptive, and both would be supported for any species in which prebreeding males grow femalelike feathers in spring. In contrast, if adult281 The Auk 103: 281-292. April 1986 This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:33:25 UTC All use subject to http://about.jstor.org/terms 282 SIEVERT ROHWER [Auk, Vol. 103 malelike feathers are grown in a spring molt, subadult plumages retained into the breeding season might more reasonably be viewed as a maladaptive constraint resulting from (1) some advantage of a subdued first winter plumage that overrides their breeding season cost, and (2) an inability to undergo a complete body molt