Within-brood size differences, sex and parasites determine blood stress protein levels in Eurasian Kestrel nestlings.
1 Brood hierarchies established through hatching asynchrony are supposed to be costly for small chicks because of impaired growth and survival. An additional cost that has remained unexplored is the stress imposed by competition for resources in the nest.
2 In the present study of broods of Eurasian kestrels, we have used the level of heat shock proteins such as Hsp60 and Hsp70 in peripheral blood as well as the heterophile/lymphocyte ratio to detect stress in nestlings. The sex of nestlings and their Caryospora sp. oocyst excretion were included in analyses.
3 Nestlings showing a large size difference with respect to their largest sibling had higher levels of both stress proteins, and this effect was stronger for female chicks as indicated by a significant interaction sex × size difference, presumably because of their higher food requirements for growth. Nestlings for which the largest sibling was a female had higher levels of Hsp60 than when it was a male. The heterophile/lymphocyte index was a much poorer predictor of competitive stress.
4 Stress proteins are effective estimators of competitive, nutritional and parasite‐mediated stress of nestlings in the wild. The cost of sustained stress has to be included in future analyses of the fitness repercussions of dominance hierarchies in avian broods.