Red coloration varies with dietary carotenoid access and nutritional condition in kittiwakes

Carotenoid-based ornaments are common signaling features in animals. Although the mechanisms that link color-based signals to individual condition is key to understanding the evolution and function of these ornaments, they are most often poorly known. Several hypotheses have been posited. They include (i) the role of foraging abilities on carotenoid acquisition and thereby carotenoid-based ornaments, and (ii) the role of internal processes linked to individual quality on the allocation and conversion of carotenoids in integuments. Here we tested the influence of dietary carotenoid access vs. internal process on gape coloration in black-legged kittiwakes (Rissa tridactyla). This seabird displays vibrant red gape, whose coloration varies with individual quality in males and is due to the deposition of red ketocarotenoids, such as astaxanthin. We decreased hydroxy- and ketocarotenoid levels in plasma, but increased efficiency in internal processes linked to nutritional condition, by supplementing breeding males with capelin, a natural energy-rich fish prey. We found that, despite having lower carotenoid levels in plasma, supplemented birds developed redder coloration than control birds, but only in the year when dietary levels of astaxanthin in the natural diet was low. In contrast, in astaxanthin-rich years, supplemented males had less-red gape than unsupplemented birds. These results suggest that inter-individual differences in internal processes may be sufficient to maintain the honesty of gape coloration under conditions of low dietary astaxanthin levels. Nonetheless, when inter-individuals variations in dietary astaxanthin levels are elevated (such as in the crustacean-rich year), carotenoid access seems a more limiting factor to the expression of gape coloration than internal processes. Our study revealed therefore a complex mechanism of gape color production in kittiwakes, and suggests that the main factor maintaining the condition-dependency of this ornaments may vary with environmental conditions and diet composition.