Delayed Maturation in Passerine Plumages and the Deceptive Acquisition of Resources

Abstract

We propose that the female-like plumage worn by some male birds in their first potential breeding season has evolved to facilitate breeding when 1-yr old through the deception of older males. By mimicking females 1-yr-old males exploit the tendency of old males not to attack females and, thus, are able to enter better quality habitats. Once subadults settle in such habitats, they hold territories in them by site dominance. An hypothesis invoking deception seems necessary because the cryptic hypothesis, which suggests that subadults are hiding from predators, cannot explain why subadult males of sexually dichromatic species always resemble females in every way by which they differ from older males. If the cryptic hypothesis were sufficient, subadult males should resemble juveniles in species for which the juvenile plumage is more cryptic than that of breeding females. Female mimicry should evolve (1) when competition for breeding resources is rigged against 1-yr-old males, (2) when obtaining a mate depends upon holding female-worthy resources, and (3) when females are limiting to males. Fitness set models suggest that individual variation in subadult appearance is evolutionarily stable only when broad gradients are established between high and low quality habitats. Using North American passerine birds, we have confirmed the following predictions from this theory. (1) Spring arrival times for 1-yr-old and older males should be nonoverlapping in species with subadult plumages. (2) Where a subadult plumage exists 1-yr-old males should more often be good mimics of females than intermediate in appearance. (3) Female limitation will favor the evolution of female mimicry in 1-yr-old males. (4) High variance in habitat quality favors subadult plumages; this holds both within and between species. (5) When a habitat gradient is abrupt and there is enough optimal breeding habitat, subadults must be better female mimics than when the gradient from good to poor habitat is gradual.