Structural and melanin coloration indicate parental effort and reproductive success in male eastern bluebirds

Male eastern bluebirds (Sialia sialis) have two types of ornamental plumage coloration: a brilliant blue-ultraviolet head, back, and wings, and a patch of chestnut breast feathers. The blue-UV coloration is produced from feather microstructure, whereas the chestnut coloration is produced by a combination of pheaomelanin and eumelanin pigments deposited in feathers. We tested the hypothesis that plumage coloration reflects male quality in eastern bluebirds, a socially monogamous, sexually dichromatic bird. We investigated whether male ornamentation correlates with mate quality and parental effort. We quantified three aspects of male ornament coloration: (1) size of the patch of chestnut breast feathers, (2) reflectance properties of the chestnut plumage coloration, and (3) reflectance properties of the blue-ultraviolet plumage coloration. We found that males with larger breast patches and brighter plumage provisioned nestlings more often, fledged heavier offspring, and paired with females that nested earlier. Males with plumage coloration that exhibit more ultraviolet hues fledged more offspring. These results suggest that plumage coloration is a reliable indicator of male mate quality and reproductive success. Both melanin-based and structural-based plumages appear to be honest signals of male quality and parental care that can be assessed by competitors or by potential mates.