Patterns of serum carotenoid accumulation and skin colour variation in kestrel nestlings in relation to breeding conditions and different terms of carotenoid supplementation.

Carotenoids are pigments synthesised by autotrophic organisms. For nestlings of raptorial species, which obtain carotenoids from the consumption of other heterotrophic species, the access to these pigments can be crucial. Carotenoids, indeed, have fundamental health maintenance functions, especially important in developing individuals as nestling kestrels. The aim of this study was to investigate how body carotenoid levels and skin pigmentation vary in kestrel nestlings (Falco tinnunculus) in relation to nesting parameters. Furthermore, we experimentally altered carotenoid availability (short- medium- and long-term) for nestlings and investigated skin and serum variance. The skin colour variance of 151 nestlings was explained by nest of origin, age and by the body condition (body mass corrected by age), older nestlings with higher body condition being redder. No difference in skin colour was detected between sexes. Differences in hue (skin "redness") between treatments did not emerge during the first week, but did occur 15 days after administration between long-term supplemented and control chicks. In contrast, the serum carotenoid concentration showed a treatment-dependent increase after 5 days from the first carotenoid administration and at least after two supplemented feedings. In general, hue but not serum carotenoids, was correlated with the body condition of nestlings. Based on the increased skin pigmentation of nestling kestrels in the long-term experimental group, we suggest carotenoid availability to be limited for colour expression. The small increase of serum carotenoids due to supplementation is consistent with the hypothesis that there is a physiological constraint on these pigments, as well as an environmental limitation. The presented results are useful for the understanding of carotenoid uptake and accumulation by a wild raptorial species, located at the top of the food web, highlighting that carotenoids are a limited resource for kestrel nestlings.