Carotenoid limitation and allocation priorities in asynchronous raptor nestlings

Abstract

The evolution and maintenance of conspicuous animal traits and communication signals have long fascinated biologists. Many yellow-red conspicuous traits are coloured by carotenoid pigments, and in some species they are displayed at a very young age. In nestling birds, the functions and proximate mechanisms of carotenoid-pigmented traits are probably different and not as well known as those of adults. Here we investigated how Montagu's harrier (Circus pygargus) nestlings within structured families used a limited resource, carotenoid pigments, and whether they used these for increasing coloration (deposition in integuments) or for mounting a response to a phytohaemagglutinin (PHA) challenge, which measures pro-inflammatory potential and aspects of cellular immune responsiveness. We manipulated carotenoid availability, using dietary carotenoid supplementations, and show that when supplemented, nestlings primarily allocated supplemental carotenoids to increase their coloration, irrespective of their sex, but depending of their position within the brood. Responses to PHA challenge were condition-dependent, but depending on carotenoid availability. Moreover, how nestlings allocated carotenoids depended on their rank within the brood, which in turn influenced their level of carotenoid limitation (first-hatched nestlings being less constrained than later-hatched nestlings). We discuss why nestlings would use supplemental carotenoids for increasing bare parts coloration rather than for responding to a PHA challenge, and the potential benefits for doing so in a parent-offspring communication context.