Quantifying conspicuousness and sexual dimorphism of the plumage in birds: a new approach

Abstract

In studies of sexual selection in birds, it is necessary to quantify the conspicuousness and sexual dimorphism of the plumage. We present a method, based on the Munsell colour system, that is easier to use than spectroradiometric techniques. It is based on measurable and repeatable data, although it is dependent on human vision. It provides indices of sexual dimorphism and incorporates aspects of close-range and long-distance conspicuousness. Ten species of Emberezidae were chosen to illustrate the procedure, and it was tested using naive observers. It consists of dividing the body of a bird into regions and matching the colour of each region to chips in a colour guide. The following indices based on the parameters of the Munsell system were selected: value (V), chroma (C), and Munsell power (V∙C), which measure conspicuousness independently of the background; Wv, Wc, and Wv∙c, which measure the contrast between the colours in the plumage; Bv, Bc, and Bv∙c, which measure the contrast between the bird and its background; and Dv, Dc, and Dv∙c, which are the indices of sexual dimorphism of the plumage. There was a high degree of agreement among the naive observers, and the scores obtained using field guides did not differ significantly from those recorded on museum specimens. One of the dimorphism indices (Dv∙c) was found to give more information than the others. However, our results indicate that conspicuousness and dimorphism indices should not be used individually because they are interdependent, and that they should be used in conjunction with multivariate analyses. Also, the proposed method incorporates long-distance conspicuousness into the results.