Predators and microorganisms of prey: goshawks prefer prey with small uropygial glands

Summary 1.  The uropygial gland of birds produces chemical substances with antimicrobial properties that have been shown to reduce the abundance of feather degrading bacteria and other microorganisms. These microorganisms would affect the flight capabilities of birds and, consequently, a relationship between size of uropygial glands and probability of capture by aerial predators should exist. 2.  We tested this hypothesis by estimating the susceptibility of 56 species of prey of the goshawk Accipiter gentilis Linnaeus to predation as the observed abundance of prey relative to the expected abundance from mean population density. 3.  In a comparative analysis of the relationship between relative size of the uropygial gland and susceptibility to predation we found a strong negative relationship accounting for 16% of the variance. This relationship was present in analyses that accounted for similarity due to common phylogenetic descent, the fact that prey of intermediate size were preferred, and that larger prey species have larger uropygial glands. 4.  These observations are consistent with uropygial glands being under strong selection from aerial predators that are likely mediated by the effect of uropygial glands on feather degrading bacteria and therefore on flight capabilities of birds.