Female plumage spottiness signals parasite resistance in the barn owl (Tyto alba)

The hypothesis that extravagant ornaments signal parasite resistance has re ceived support in several species for ornamented males but more rarely for ornamented females. However, recent theories have proposed that females sho uld often be under sexual selection, and therefore females may signal the h eritable capacity to resist parasites. We investigated this hypothesis in t he socially monogamous barn owl, Tyto alba, in which females exhibit on ave rage more and larger black spots on the plumage than males, and in which ma les were suggested to choose a mate with respect to female plumage spottine ss. We hypothesized that the proportion of the plumage surface covered by b lack spots signals parasite resistance. In line with this hypothesis, we fo und that the ectoparasitic fly, Carnus hemapterus, was less abundant on you ng raised by more heavily spotted females and those flies were less fecund. In an experiment, where entire clutches were cross-fostered between nests, we found that the fecundity of the flies collected on nestlings was negati vely correlated with the genetic mother's plumage spottiness. These results suggest that the ability to resist parasites covaries with the extent of f emale plumage spottiness. Among females collected dead along roads, those w ith a lot of black spots had a small bursa of Fabricius. Given that parasit es bigger the development of this immune organ, this observation further su ggests that more spotted females are usually less parasitized. The same ana lyses performed on male plumage spottiness all provided non-significant res ults. To our knowledge, this study is the first one showing that a heritabl e secondary sexual characteristics displayed by females reflects parasite r esistance.