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Background  Animals use carotenoid-pigments for coloration, as antioxidants and as enhancers of the immune system. Carotenoid-dependent colours can thus signal individual quality and carotenoids have also been suggested to mediate life-history trade-offs.    Methodology  To examine trade-offs in carotenoid allocation between parents and the young, or between skin coloration and plasma of the parents at different levels of brood demand, we manipulated brood sizes of Eurasian kestrels ( Falco tinnunculus ).    Principal Findings  Brood size manipulation had no overall effect on plasma carotenoid levels or skin hue of parents, but female parents had twice the plasma carotenoid levels of males. Males work physically harder than females and they might thus also use more carotenoids against oxidative stress than females. Alternatively, females could be gaining back the carotenoid stores they depleted during egg-laying by eating primarily carotenoid-rich food items during the early nestling stage. Fledglings in enlarged broods had higher plasma carotenoid concentrations than those in reduced broods. This difference was not explained by diet. In light of recent evidence from other species, we suggest it might instead be due to fledglings in enlarged broods having higher testosterone levels, which in turn increased plasma carotenoid levels. The partial cross-foster design of our experiment revealed evidence for origin effects (genetic or maternal) on carotenoid levels of fledglings, but no origin-environment interaction.    Significance  These results from wild birds differ from studies in captivity, and thus offer new insights into carotenoid physiology in relation to division of parental care and demands of the brood.

Effects of Experimental Brood Size Manipulation and Gender on Carotenoid Levels of Eurasian Kestrels Falco tinnunculus