ADAPTIVE SIGNIFICANCE OF DELAYED PLUMAGE MATURATION IN MALE NORTHERN ORIOLES.

Abstract

The Northern Oriole (Icterus galbula) is one of 105 sexually dichromatic species of passerine bird in North America. Like 29 other species in this group, it exhibits delayed male plumage maturation: although reproductively mature during their first potential breeding season (Flood, 1980; Sealy, 1980), male Northern Orioles do not acquire fully adult breeding plumage until the following year. Females, which do attain adult appearance when less than a year old, are variable in coloration. Individual females may be bright, although most are much duller than the brilliantly-colored black and orange adult males. During their first potential breeding season males possess a variable plumage that is, on average, intermediate between that of females and older males. The appearance of females and first-year males overlaps considerably, however, and it is impossible to determine with certainty the sex of a dullcolored bird on the basis of plumage alone. Delayed plumage maturation poses an interesting evolutionary problem. If, as is thought, the bright coloration of males in some way facilitates their ability to breed, what is the advantage to first-year birds of looking like females? Two hypotheses have been proposed to explain the adaptive significance of delayed male plumage maturation in passerines. The first, suggested by Lack (1954) and developed in more detail by Selander (1965, 1972) has been termed the Sexual Selection Hypothesis (SSH) (Wittenberger, 1978; Rohwer et al., 1980). The basic premise of the SSH is that in certain situations, the life-time fitness of males is increased if reproduction is deferred until after the first potential breeding season. In some taxa, the intensity of male aggression may exclude young, inexperienced males from breeding. Also, colorful males-especially while displaying-must be exposed to greater risk of predation than ones that are cryptic in plumage and action. According to Selander (1965, 1972), first-year males should be selected to minimize their conspicuousness if there is little chance for them to breed. This could be achieved by the adoption of a cryptic plumage and the avoidance of competition and courtship during the first year. More recently, Rohwer and others have advanced an alternative explanation for delayed maturation which they term the Female Mimicry Hypothesis (FMH) (Rohwer, 1978; Rohwer and Niles, 1979; Rohwer et al., 1980). In contrast to the SSH, the FMH suggests that delayed maturation is not a handicap to, but actually facilitates, breeding by first-year males. The dull coloration of yearling males allows them to mimic females and thereby deceive adult males; it allows them to exploit the tendency of such males to defer to females. By this deceit young males may establish residency within or near the high quality territories of adult males, and acquire sufficient site dominance (Maynard Smith, 1974; Parker 1974) to be able to retain a space for breeding, and attract a female. Although the FMH and the SSH are not mutually exclusive, they do give rise to a number of differing predictions. In this study I investigate the adaptive significance of the delay in acquisition of bright plumage by male Northern Orioles, and test various predictions arising