The Function of Delayed Plumage Maturation in Male Black-Headed Grosbeaks

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Abstract
I used data from two breeding populations of Black-headed Grosbeaks (Pheucticus melanocephalus) in central New Mexico to test alternative hypotheses regarding the function of delayed plumage maturation in male passerines. Yearling male grosbeaks displayed a wide range of subadult plumage types, but most individuals were intermediate in appearance between adult males and females. Subadult male grosbeaks arrived on the study sites about 2 weeks after adult males, with no tendency for individuals with brighter or duller plumage to arrive first. Only a few of the most brightly plumaged subadult males defended territories and attracted females; most were nonterritorial floaters. All territories of subadult males were positioned outside clusters of adult male territories with few total neighbors. When I removed adult males from their territories, the territories remained empty. These results are not consistent with predictions of the female-mimicry hypothesis, but they are in accord with those of the cryptic hypothesis.

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