Blue males and green females. Sexual dichromatism in the swallow-tanager (Tersina viridis) and the blue dacnis (Dacnis cayana).

BARREIRA, A.; GARCÍA, G.; LOUGHEED, S. C.; TUBARO, P. L.

ORNITOLOGIA NEOTROPICAL

Neotropcal Ornithological Society, USA

Año: 2008 vol. 19 p. 441 - 450

1075-4377

Abstract. – Sexual differences in plumage coloration, referred to as sexual dichromatism, are a common phenomenon among tanagers (family Thraupidae). In many species of this group, sexual dichromatism is perceived, by human observers under natural light conditions, as blue males and green females. For instance, this pattern is recurrent among species of several genera, such as Chlorochrysa, Tangara, Dacnis, Chlorophanes, Cyanerpes and Tersina. To understand the physical nature of this phenomenon, and to assess the degree of similarity/difference in sexual dichromatism exhibited by different species, we described the plumage color of the Swallow Tanager (Tersina viridis) and the Blue Dacnis (Dacnis cayana) using reflectance spectrophotometry. In both species there were no sexual differences in hue. Instead, males showed a secondary peak of reflectance in the short-wavelength (UV-violet) part of the spectrum, which was absent in females. This bimodal reflectance shape of males is different from the most commonly found unimodal pattern of blue plumages. Given the spectral similarity by which both studied species achieve the same blue-green sexual dichromatism, and the fact that molecular evidence indicates a close relationship between the genera Tersina and Dacnis, we interpret the particular sexual dichromatism found in both the Swallow Tanager and the Blue Dacnis as a homologous condition. Accepted 3 June 2008.