Female Ornamentation in the American Robin
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Ornamental traits in male birds have been the subject of much research effort, and sexual selection is recognized as the leading explanation for their evolution. The expression of ornamental traits in females has received little study until recent decades. Female colouration has been considered a non-adaptive, correlated response to selection on males. However, models predict that male mate choice, female competition, and the evolution of honest signals could help explain female ornamentation, especially where male investment in offspring and variation in female quality are high. I investigated this in the American robin (Turdus migratorius), a socially monogamous species with bi-parental care and variable female ornamentation. Female robins display conspicuous red breast plumage, bright yellow bills, and achromatic ornamentation. Female ornamentation is similar to males, but is subdued to varying degrees across individuals. Female colouration could function as a useful criterion in mate selection by males if it is correlated with aspects of female quality important to producing viable offspring. I assessed whether female ornamentation in robins might act as an honest signal by relating variation in female colour to measures of individual quality and reproductive investment. To assess ornamentation, I took colour measurements of the bill, crown and breast of male and female robins in the field using reflectance spectrometry. Female bill, breast and crown traits reliably predicted age, crown and bill colour traits were related to better body condition, and bill colour decreased seasonally as well as with ectoparasite load. I found evidence of assortative mating based on crown UV reflectance and bill colour. To assess reproductive investment, I measured egg size, yolk proportion, and deposition of yolk testosterone and carotenoids. Females with brighter (lighter) carotenoid-based bill colour laid larger eggs, and females with yellower bills laid eggs with higher yolk proportions and more total yolk carotenoids. Yolk testosterone level was associated with redder female breasts. These results support the hypothesis that female colour may be a reliable indicator of individual quality and capacity for reproductive investment.