Condition–dependent variation in the blue–ultraviolet coloration of a structurally based plumage ornament

Abstract

After years of investigation into the function of sexually dimorphic ornamental traits, researchers are beginning to understand how bright plumage colour in birds acts as an intraspeci¢c signal. This work has focused primarily on pigment-based ornaments because they are highly variable in patch size, hue and brightness for some species. In contrast, structurally based ornaments have been little studied, in part because they do not appear to be as variable as pigment-based ornaments. We investigated a structurally based plumage ornament in a wild population of blue grosbeaks (Guiraca caerulea), a sexually dimorphic passerine. We report plumage variation that extends into the ultraviolet region of the spectrum. The pattern of covariation between four out of ¢ve elements of plumage variation suggests that structurally based ornamentation is pushed towards extreme expression of the trait as predicted by the sexual selection theory. The`bluest' birds have the highest percentage of blue feathers on the body. These ornamental feathers re£ect light maximally at the shortest wavelengths (ultraviolet), with the greatest intensity and the greatest contrast. Age may have some e¡ect on expression of blueness. In addition, plumage variables are correlated with growth bars in tail feathers (a record of nutritional condition during moult in a nonornamental trait). This suggests that the ornament is partially condition dependent. Thus, blue plumage in male grosbeaks may serve as an honest indicator of age and quality.

Egg colour matching in an African cuckoo, as revealed by ultraviolet-visible reflectance spectrophotometry

“…This should not be a surprise as such undulations have been observed in feathers of a variety of species (e.g. Hunt et al 1998;Keyser & Hill 1999), including those measured by di¡erent groups and using di¡erent spectrophotometers, and in eggs of other species (A. T. D. Bennett, unpublished results). Inspection of the mean spectra shows that host eggs re£ect more at all wavelengths than cuckoo eggs, a result supported by later analyses.…”

Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly,Colias eurytheme

“…Investigations of colouration as honest signals of quality have focused primarily on pigment-based colouration (Hamilton & Zuk 1982;Lozano 1994;Olsen & Owens 1998;Hill 1999;McGraw & Hill 2000;Grether et al 2001;Blount et al 2003), but structural colours are being studied with increasing frequency from this perspective ( Johnsen et al 1998( Johnsen et al , 2003Keyser & Hill 1999;McGraw et al 2002;Doucet & Montgomerie 2003;Siefferman & Hill 2003). Most animal colours, however, are generated through a combined effects of these mechanisms.…”

Ultraviolet reflectance by the cere of raptors

“…Badyaev & Hill 2000;Møller et al 2000) and structural plumage coloration (e.g. Andersson 1999;Keyser & Hill 1999) have received particular attention. Experiments have also shown that many species detect near-ultraviolet light (UV; 300-400 nm) in addition to the spectrum visible to humans (400-700 nm; Cuthill et al 2000) and can use UV signals for quality appraisal and mate choice (e.g.…”