Assortative mating by multiple ornaments in northern cardinals (Cardinalis cardinalis)

Abstract

In positive assortative mating, individuals of similar phenotype mate together more frequently than expected by chance. Assortative mating by a variety of qualities, including ornamentation, is well documented in birds. Studies of assortative mating by ornaments have focused on single, highly conspicuous ornaments, but many species of birds possess multiple ornaments in both sexes. We compared ornament expressions between mates of northern cardinals (<it>Cardinalis cardinalis</it>) to determine if assortative mating occurred by one or more of the four ornaments displayed by both sexes. All cardinals possess tall head crests and red-orange bills. In addition, males have black face masks and entirely red body plumage, whereas females have blackish face masks and red underwing coverts. We predicted that cardinals mate assortatively by plumage color because red plumage expression has been shown to indicate quality in both sexes. We found that cardinals mate assortatively by plumage and bill color, the two ornaments colored by carotenoid pigments, but not by mask expression or crest length. Whether this mating pattern arises by mutual mate choice or intrasexual selection is not known.