Abstract
Like males of approximately 30 other species of North American passerines, male Black-headed Grosbeaks (Pheucticus melanocephalus) do not attain complete definitive alternate plumage until their second breeding season. I collected yearling and adult male Black-headed Grosbeaks in the Rio Grande valley in central New Mexico to test the hypothesis that delayed plumage maturation in Black-headed Grosbeaks is one manifestation of a general, hormonally-mediated developmental pattern. I compared the mass of testes to the degree of plumage maturation in eleven yearling males and found a significant positive correlation. Moreover, the mean testis mass of yearling males was significantly lower than the mean testis mass of older males. In contrast, I found no significant relationship between plumage brightness and testis mass among seven adult males in definitive alternate plumage. These observations support the idea that delayed plumage maturation in Black-headed Grosbeaks is part of a general delay in investment in first-year reproduction.
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