Monochromatism, cryptic sexual dimorphism and lack of assortative mating in the Rufous Hornero, Furnarius rufus albogularis

Abstract

Neotropical ovenbirds (family Furnariidae) are largely sexually monomorphic and monochromatic, which leads to the assumption that sexual selection has had little effect on the evolution of the morphological and plumage traits of the species in the family. We studied a wild population of the Rufous Hornero (Furnarius rufus albogularis) and used morphological measurements, molecular sexing, spectrometer analyses and visual modelling to investigate the assumption of sexual monomorphism and monochromatism in this species. We also tested for assortative mating with respect to these traits. On average, males had slightly longer wings and tails than females but there were no sexual differences in other morphological traits (mass, tarsus and bill) or in the spectral properties of plumage coloration for six body parts. Visual modelling indicated that Rufous Horneros can perceive variation in colour between individuals but colour does not vary with sex. We did not find any evidence of assortative mating for size or colour traits. In conclusion, males from the studied population differ slightly from females in external morphological measurements but not in plumage coloration. This study is among the first to demonstrate complete sexual monochromatism in birds assessed against the avian visual system.