Assortative mating in relation to plumage traits shared by male and female American Robins

Whereas the ecological and evolutionary causes and consequences of sexual dimorphism in birds have been widely explored, the functional significance of female ornamentation in mutually ornamented species is less well understood. One hypothesis for mutual ornamentation is mutual sexual selection, where similar selective pressures shape the ornamental traits in both sexes. We examined multiple melanin-based plumage traits (chestnutcolored breast plumage, tail-spot size and damage) in male and female American Robins (Turdus migratorius) in relation to patterns of mating, individual quality, breeding performance, and incidence of extra-pair paternity. We found positive assortative social mating based on breast color and tail-spot damage. Tail-spot size was unrelated to patterns of social mating, and none of the plumage traits examined was associated with the incidence of extra-pair paternity. Brighter breast plumage and less damage to tail spots were positively associated with several measures of breeding performance in female American Robins. In males, body condition was positively correlated with breast color, but nest defense was actually lower in males with brighter breasts and less tail-spot damage. Although male and female ornamentation in the American Robin may be the product of mutual mate choice, further assessment of this hypothesis requires information on how an individual's parental investment is affected by its own and its mate's plumage and on how plumage varies with age.