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.

Evidence suggests that structural plumage colour can be an honest signal of individual quality, but the mechanisms responsible for the variation in expression of structural coloration within a species have not been identified. We used full–spectrum spectrometry and transmission electron microscopy to investigate the effect of variation in the nanostructure of the spongy layer on expression of structural ultraviolet (UV)–blue coloration in eastern bluebird (Sialia sialis) feathers. Fourier analysis revealed that feather nanostructure was highly organized but did not accurately predict variation in hue. Within the spongy layer of feather barbs, the number of circular keratin rods significantly predicted UV–violet chroma, whereas the standard error of the diameter of these rods significantly predicted spectral saturation. These observations show that the precision of nanostructural arrangement determines some colour variation in feathers.

Although the function of ornamental traits in males has been the focus of intensive research for decades, expression of such traits in females has received much less study. Eastern bluebirds (Sialia sialis) display structurally based ultraviolet/blue and melanin-based chestnut plumage, and in males this plumage coloration is related to both reproductive success and competitive ability. Compared to males, female bluebirds show a subdued expression of blue and chestnut ornamental coloration, and we used a combination of an aviary nutritional-stress experiment and four years of field data to test the hypothesis that coloration functions as a signal of female quality. First, we tested the effect of food intake on expression of structural and melanin coloration in female eastern bluebirds to determine whether structural or melanin coloration are condition-dependent traits. Females that were given ad libitum access to food displayed more ornamented structural coloration than females on a food-restricted diet, but there was no effect of the experiment on melanin ornamentation. Second, we used field data to assess whether female ornamentation correlated with measures of mate quality and parental effort. The structural coloration of females predicted first egg date, maternal provisioning rates, and measures of reproductive success. These data indicate that structural coloration is dependent on nutritional condition and suggest that sexual selection is acting on structurally based plumage coloration in female eastern bluebirds.

Life-history theory proposes that organisms must trade-off investment in current and future reproduction. Production of ornamental display is an important component of reproductive effort that has rarely been considered in tests of allocation trade-offs. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet-blue plumage that is correlated with mate acquisition and male competitive ability. To investigate trade-offs between current reproductive effort and the future expression of a sexually selected ornament, we manipulated the parental effort of males by changing their brood sizes. We found that parents provisioned experimentally enlarged broods more often than reduced broods. As predicted by life-history theory, the change in parental effort had a significant effect on the relative plumage ornamentation of males in the subsequent year: males with reduced broods significantly increased in plumage brightness. Moreover, this change in plumage coloration had a direct effect on the timing of breeding in the following season: males that displayed brighter plumage in the year following the manipulation mated with females that initiated egg laying earlier in the season. These data indicate that male bluebirds must trade-off conserving energy for production of future ornamentation versus expending energy for current reproduction.