Male and female birds can exhibit striking differences in morphology (dimorphism), including exaggerated plumage structures, size differences, and feather color differences, yet most species are considered to have identical sexes. Classification of a species as sexually monochromatic (identical coloration between the sexes) has traditionally been based on human visual perception. Research over the past two decades has shown that birds see each other and the world differently than their human investigators; these visual differences, combined with objective color measurements, suggest that traditional assessments of plumage coloration may be questionable from an avian visual perspective (Cuthill et al. 1999). I tested the avian reality of human categorization of sexually monochromatic plumages for North American passerines. I found, on the basis of plumage-reflectance data and modeled avian color-discrimination thresholds, that 91.6% of the presumed sexually monochromatic species I sampled are sexually dichromatic to birds, which confirms the rarity of sexually monochromatic bird species. This study informs future work as to potential sources of plumage signals hidden from human observers and generates testable predictions regarding avian sex-recognition via plumage signals.

La Coloración del Plumaje desde la Perspectiva de la Visión de las Aves Confirma la Rareza del Monocromatismo Sexual en las Aves Paserinas de Norte América