Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis)

Abstract

Many of the brilliant plumage coloration displays of birds function as signals to conspecifics. One species in which the function of plumage ornaments has been assessed is the Eastern bluebird (Sialia sialis). Studies of a population breeding in Alabama (USA) have established that plumage ornaments signal quality, parental investment, and competitive ability in both sexes. Here we tested the additional hypotheses that (1) Eastern bluebird plumage ornamentation signals nest defense behavior in heterospecific competitive interactions and (2) individual variation in plumage ornamentation reflects underlying differences in circulating hormone levels. We also tested the potential for plumage ornaments to signal individual quality and parental investment in a population breeding in Oklahoma (USA). We found that Eastern bluebirds with more ornamented plumage are in better condition, initiate breeding earlier in the season, produce larger clutches, have higher circulating levels of the stress hormone corticosterone, and more ornamented males have lower circulating androgen levels. Plumage coloration was not related to nest defense behavior. Thus, plumage ornamentation may be used by both sexes to assess the physiological condition and parental investment of prospective mates. Experimental manipulations of circulating hormone levels during molt are needed to define the role of hormones in plumage ornamentation.

Effect of Prenatal and Natal Administration of Testosterone on Production of Structurally Based Plumage Coloration

“…The testosterone-treated birds preened more often in the spring than control birds; thus, preening is likely the mechanism behind this positive covariation. Overall, our data demonstrating that nestlings implanted with testosterone exhibit duller blue color corroborates the findings of a recent study of eastern bluebirds breeding in Oklahoma; adult males with less ornamented blue plumage (UV chroma and hue) had significantly lower androgen levels and higher corticosterone levels (Grindstaff et al 2012). The negative relationship that we found between testosterone treatment and the brightness of structurally based blue plumage could be either a direct consequence of testosterone on the development of feather nanostructure or an in-direct effect of the negative relationship between either testosterone or corticosterone and body condition.…”

Consistency of structural color across molts: The effects of environmental conditions and stress on feather ultraviolet reflectance

“…In species with structural coloration, plumage variation results from differences in energy allocated to feather growth due to environmental conditions during molt, where birds that grow feathers faster have brighter, more ornamented feathers (Prum 2006, Siefferman et al 2008). Habitat quality, diet, social hierarchy, and parasite levels all can affect an individual's energy levels and thus feather growth, making it an honest signal since birds with large energy budgets can invest more in feather development (Hamilton and Zuk 1982, Grubb 2006, Hill 2006, Grindstaff et al 2012. Structural color could therefore result from and signal an individual's condition or resiliency to stressors Montgomerie 2003, Grindstaff et al 2012; but see Peters et al 2011), making it a useful indicator of individual and population health.…”

“…However, evidence for correlations between CORT and UV reflectance appear to be mixed. Henderson et al (2013) observed a negative correlation between UV color and baseline CORT in female Blue Tits, but this was not the case in Eastern Bluebirds, where a positive correlation existed between CORT and UV hue and saturation (Grindstaff et al 2012). In FSJs, Tringali and Bowman (2012) found that UV reflectance signals dominance in juveniles, and that the relative change in dominance was positively correlated with the relative difference in CORT (Tringali, pers.…”

Corticosterone regulates multiple colour traits in Lacerta [Zootoca] vivipara males

“…These studies support that melanin-based traits can also function as condition-dependent traits and, therefore, that different types of colours might share common evolutionary mechanisms. In this regard, recent studies investigating single ornaments of different species found that corticosterone levels (the main glucocorticoid released during stressful conditions in vertebrates; Wingfield et al, 1998) are linked to the expression of different types of colorations (Calisi & Hews, 2007;Loiseau et al, 2008;Roulin et al, 2008;Fitze et al, 2009;Lobato et al, 2010;Grindstaff et al, 2012;Mart ınez-Padilla et al, 2013). This suggests that stress hormones, which negatively affect condition in situations of long-term stress (Moore & Miller, 1984;Tokarz, 1987;Dhabhar & McEwen, 1997;Cote et al, 2010), may account for condition dependence of multiple types of ornaments in the same individual.…”