Garden Warbler, Sylvia borin, detect carotenoids in food but differ strongly in individual food choice

The concept of optimal foraging has been used to show how animals can increase their fitness by actively regulating the ingestion of nutrients or by avoiding detrimental plant metabolites, such as alkaloids. There is, however, considerably less information available on whether and, if so, how animals actively select beneficial secondary compounds, such as dietary antioxidants. In the last two decades, carotenoids have been the most intensively studied dietary antioxidants in behavioural ecology, and the expression of carotenoid-based ornamentation has become a model for studying the condition dependence of sexually selected traits. In a series of experiments in which we offered individual Garden Warblers (Sylvia borin) a choice between two foods, one with and one without carotenoids, we found that individual birds did not select food for the maximum amount of carotenoids, rather they choose for a highly consistent carotenoid intake during the course of different dual-choice experiments in which the food offered differed in carotenoid content. However, it remains unclear whether the individual birds were able to optimise their carotenoid intake and, therefore, the expression of life history traits—even though individuals differed consistently in their carotenoid intake. In particular, females tended to avoid food with high carotenoid contents. Based upon the highly consistent carotenoid intake of individual birds, we conclude that our birds were able to perceive the presence of carotenoids in the food without the use of visual cues. We suggest that these results may shed new light on the mechanisms that birds may use to regulate the ingestion of these antioxidants and, consequently, possibly affect the expression of life history traits, such as immunocompetence or sexually selected traits.