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Abstract 

Species recognition is critical for maintaining species barriers by preventing interbreeding between distinct species that co-occur and regularly interact. In birds, feather ornamentation is a prominent mechanism underlying species recognition, where differences in feather coloration and patterning (i.e., plumage) regulate reproductive transactions within, and also between, species. To explore how plumage might regulate interspecific interactions, this study quantified the plumage variation between and within two closely-related, interbreeding songbirds from an avian perspective. Understanding how plumage color and patch size vary between and within black-capped and mountain chickadees will reveal what plumage patches might be used for species recognition and whether plumage might be used in intraspecific reproductive transactions. To quantify plumage from an avian perspective, I photographed museum specimens in visible and ultraviolet light for color and patch size analysis and measured the color of feathers collected from wild chickadees using a spectrophotometer. I found that black-capped chickadees had warmer, brighter plumage with more color contrast and smaller plumage patches than mountain chickadees. Within either species, plumage color nor patch size differed between the sexes, indicating that both species are sexually monochromatic and monomorphic from an avian perspective. Broadly, plumage might be an important social signal used in species recognition but potentially not in intraspecific reproductive transactions. Evidence for sexual monochromatism in avian species correlates with high intelligence, suggesting that cognitive abilities might be more important for sexual selection than plumage. Keywords: hybridization, plumage, sexual monochromatism, species recognition, tetrachromatic