Plumage and sexual maturation in the Great Frigatebird Fregata minor in the Galapagos Islands

The adaptive significance of distinctive immature plumages and protracted sexual and plumage maturation in birds remains controversial. This study aimed to establish the pattern of plumage maturation and the age at first breeding in the Great Frigatebird Fregata minor in the Galapagos Islands. We found that Great Frigatebirds attain full adult plumage at eight to nine years for females and 10 to 11 years for males and that they rarely attempted to breed before acquiring full adult plumage. The younger males succeeded only at attracting a mate, and males and females both bred at the age of nine years when their plumage was nearly completely adult. Although sexual maturity was reached as early as nine years, strong competition for nest-sites may further delay first reproduction. We discuss our findings in light of the several hypotheses for explaining delayed plumage maturation in birds, concluding that slow sexual and plumage maturation in the Great Frigatebird, and perhaps among all frigatebirds, may result from moult energetic constraints during the subadult stage. Therefore, slow plumage maturation in frigatebirds may not be an adaptation for distinctive subadult plumage per se, as is often suggested for birds.