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

In positive assortative mating, individuals of similar phenotypemate together more frequently than expected by chance. Assortativemating by a variety of qualities, including ornamentation, iswell documented in birds. Studies of assortative mating by ornamentshave focused on single, highly conspicuous ornaments, but manyspecies of birds possess multiple ornaments in both sexes. Wecompared ornament expressions between mates of northern cardinals(Cardinalis cardinalis) to determine if assortative mating occurredby 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 bodyplumage, whereas females have blackish face masks and red underwingcoverts. We predicted that cardinals mate assortatively by plumagecolor because red plumage expression has been shown to indicatequality in both sexes. We found that cardinals mate assortativelyby plumage and bill color, the two ornaments colored by carotenoidpigments, but not by mask expression or crest length. Whetherthis mating pattern arises by mutual mate choice or intrasexualselection is not known.