The significance of subadult plumage in Darwin's finches, Geospiza fortis

I studied delayed plumage maturation, a common phenomenon in passerine birds, in a species of Darwin's finch, Geospiza fortis, on Isla Daphne Major, Galapagos, Ecuador. To address the question of why males advertise their youth and immaturity with subadult plumage, I determined the fitness consequences of possessing different plumage states at different ages. Rate of entry into adult plumage is both variable and heritable. In two cohorts, individuals that took longer to enter full adult plumage survived dry conditions significantly better than those that acquired adult plumage quickly. Behavioral observations helped reveal the reasons for this higher survival. Advantages arose from the avoidance of conflict that occurred by signaling subordinate status with subadult plumage in both the nonbreeding and breeding seasons. Although blacker males had a significantly lower survival, those black males that did survive had a reproductive advantage in their first breeding season over males with less black in their plumage from the same cohort and raised significantly more offspring to fledging. Thus the study demonstrates selection on the length of the subadult plumage stage. [Behav Ecol 1990;1:161-170]