Seasonal, sexual, and quality related variation in retinal carotenoid accumulation in the house finch (Carpodacus mexicanus)

1. Studies of visual ecology generally focus on the tuning of the eye to the spectral environment. However, the environment may also shape vision if the availability of nutrients or other extrinsic stressors impact eye structure or function. 2. Carotenoids are diet-derived pigments that accumulate in the retinas of birds, where they provide photoprotection and tune colour vision. In domesticated species, carotenoid accumulation in retina is dependent on dietary intake, but little is known about the variability in or control of these pigments in the eyes of wild animals. 3. Carotenoids are also deposited in the integument of many animals, where they generate colourful sexually selected displays that communicate information about individual health and nutrition. We hypothesize that retinal carotenoid accumulation is subject to the same health and nutritional constraints as the use of carotenoids in colour signals. 4. As a first test of this hypothesis, we examined retinal carotenoid accumulation in relation to season, sex, body condition, circulating plasma carotenoid concentrations, and plumage colouration in a free ranging population of house finches ( Carpodacus mexicanus ) – a model species for studies of carotenoid ecology. 5. Retinal carotenoid accumulation varied considerably among individuals and differed significantly among seasons, with the highest levels observed in late fall and winter. Body condition and plasma zeaxanthin levels were significantly positively correlated with retinal carotenoid accumulation, but retinal carotenoid concentrations did not differ between the sexes. Plumage redness covaried positively with retinal carotenoid concentration as well, though this relationship was no longer significant when accounting for seasonal variation. 6. Our results, although correlational, do suggest that retinal carotenoid accumulation is a variable trait that may be influenced by environmental and physiological conditions, raising the intriguing possibility that plumage colouration and colour vision could be linked through a common biochemical mechanism.