Female house sparrows prefer big males with a large white wing bar and fewer feather holes caused by chewing lice

Males frequently signal their resistance against parasites by elaborate ornaments. By mating with more ornamented males, females may choose less parasitized partners, and benefit by reducing the probability of contagion of parasites with direct transmission. Chewing lice (order Phthiraptera) are parasites of birds that considerably harm hosts, even decreasing survival. Previous studies showed that male house sparrows (Passer domesticus) can signal resistance against chewing lice by the size of the white wing bar, with more resistant birds having a larger bar. Here, in a mate-choice experiment, 16 female sparrows were presented to 16 dyads of males having similar initial wing bar sizes. In each dyad, the wing bar was experimentally reduced in a randomly selected male. Female house sparrows chose males with larger wing bars. Nevertheless, females also preferred males with less feather holes caused by chewing lice and larger males. By choosing males with larger wing bars, females choose males with larger uropygial glands, an organ involved in the resistance against chewing lice in this species. Therefore, white patches, which are widespread in birds, might be used by females in order to evaluate male resistance against chewing lice.