Are red-crested cardinals Paroaria coronata (Aves: Thraupidae) truly monomorphic?: evidence discloses sexual dichromatism and size differences since the nestling stage

The red-crested cardinal (Paroaria coronata) is an emblematic bird of the southern cone of South America that has been traditionally classified as sexually monomorphic and monochromatic from a human visual perspective. Using reflectance spectrometry, we analyzed sexual differences in the coloration of the red head plumage patch of adult wild red-crested cardinals in central-eastern Argentina. We also described sexual differences in body size and determined the onset of morphometric differences by comparing morphological measurements and growth rate between male and female nestlings. Adult males had on average longer wings and brighter red plumage relative to females. Similar to adults, nestlings were sexually dimorphic at fledging and presented sex-specific growth curves. Male nestlings grew faster and reached a larger size at the fledging time. Both for adults and nestlings, wing length was the trait that expressed sexual differences in body size. Our results are consistent with recent studies in passerines indicating that many birds considered sexually monochromatic are likely dichromatic when considering avian visual capacities. This study allows the red-crested cardinal to be re-categorized as sexually dimorphic and dichromatic.