Relationships between plumage coloration, diet diversity, and winter body condition in the Lesser Goldfinch

Abstract

Bright and colorful plumage is thought to be an honest signal of individual quality in birds because consuming high-quality forage results in more colorful plumage. To gain insight into the ecological and evolutionary context of the relationship between color, body condition, and diet, we studied a mostly urban population of Lesser Goldfinches (Spinus psaltria). We collected body measurements and digital photographs of plumage, as well as feathers and blood of goldfinches during three winters (2009–2012) in northern Nevada, USA. We analyzed the body tissues (feathers and blood) for stable isotope values of carbon and nitrogen to infer the diets of individual goldfinches, and quantified CIELAB color space values of chroma, brightness, and hue of plumage from the digital images. We then examined the relationships between color values and body condition, and color and stable isotope values. We found that the brightness (L* value) of the back plumage was correlated with both body condition and with stable isotope values of nitrogen (δ15N) in the winter diet. Furthermore, stable isotope analyses of both feathers and blood showed temporal differences in diet. However, hue and chroma, which are color values that are thought to more directly represent feather carotenoid content, were not related to body condition or diet. Our results suggest that the foraging ecology of Lesser Goldfinches changes over time, and that, in winter, plumage color values that are putatively indicative of carotenoid content do not seem to be an honest signal of individual quality as measured by body condition.