Avian color expression and perception: Is there a carotenoid link?

Carotenoids color many of the red, orange and yellow ornaments of birds and also shape avian vision. The carotenoid-pigmented oil droplets in cone photoreceptors filter incoming light and are predicted to aid in color discrimination. Carotenoid use in both avian coloration and color vision raises an intriguing question: is the evolution of visual signals and signal perception linked through these pigments? Here, we explore the genetic, physiological and functional connections between these traits. Carotenoid color and droplet pigmentation share common mechanisms of metabolic conversions and are both affected by diet and immune-system challenges. Yet, the timescale and magnitude of these effects differ greatly between plumage and the visual system. Recent observations suggest a link between retinal carotenoid levels and color discrimination performance, but the mechanisms underlying these associations remains unclear. Therefore, we performed a modeling exercise to ask whether and how changes in droplet carotenoid content could alter the perception of carotenoid-based plumage. This exercise revealed that changing oil droplet carotenoid concentration does not substantially affect the discrimination of carotenoid-based colors, but might change how reliably a receiver can predict the carotenoid content of an ornament. These findings suggest that, if present, a carotenoid link between signal and perception is subtle. Deconstructing this relationship will require a deeper understanding of avian visual perception and the mechanisms of color production. We highlight several areas where we see opportunities to gain new insights, including comparative genomic studies of shared mechanisms of carotenoid processing and alternative approaches to investigating color vision.This dataset includes house finch plumage spectrum measures and feather carotenoid content measures collected following methods in: Butler, M., Toomey, M. and McGraw, K. (2011). How many color metrics do we need? Evaluating how different color-scoring procedures explain carotenoid pigment content in avian bare-part and plumage ornaments. Behavioral Ecology and Sociobiology 65, 401–413; 413. House finch cone oil droplet absorbance spectra collected following methods in: Toomey, M. B., Collins, A. M., Frederiksen, R., Cornwall, M. C., Timlin, J. A. and Corbo, J. C. (2015). A complex carotenoid palette tunes avian colour vision. J. R. Soc. Interface 12, 20150563. Irradiance spectra, ocular media transmittance spectra, and visual system parameters used in the modeling of avian visual performance. Also included are example R-scripts and R.data files to run the analyses presented in the associated publication