Plumage characteristics, reproductive investment and assortative mating in tree swallows Tachycineta bicolor

Elaborate ornamental plumage has been associated with various measures of individual quality in many species of birds. Male plumage characteristics, which have been relatively well studied, have been shown to reflect past reproductive investment, as well as the potential for reproductive investment in the current breeding attempt. In contrast, the signalling functions of female traits remain largely unexplored. In this study, we investigated the relationship between plumage attributes of breeding adult tree swallows and past reproductive investment, current reproductive investment and social mate pairing strategy. Both males and older females possess metallic green to metallic blue iridescent plumage on their dorsal surface, making this a suitable species for this type of investigation. We did not find any effects of past reproductive investment and success on the plumage attributes of returning breeders. In contrast, female plumage hue covaried with fledging success, and female plumage brightness was positively associated with mean clutch egg mass. In addition, we found that social pairs mated assortatively with respect to plumage brightness. We argue that since plumage characteristics vary with age in both male and female tree swallows, plumage attributes in this species are indicative of breeding experience and may be honest signals of quality. Positive assortative pairing could be the result of mutual mate choice or intra-sexual competition for nest sites by both males and females.