Sexual selection in the house sparrow, Passer domesticus

(1) This study investigated the maintenance of variation in the black throat patch or 'badge' of the male house sparrow. This sexually dimorphic trait is thought to be a sexually selected ornament, with previous workers providing evidence of a role in both mate choice of males by females and male-male competition. The study was conducted in 1995 and 1996 in a closed population of approximately 40 breeding pairs on Lundy Island, in the Bristol Channel, England.;(2) Genetic analysis of paternity using PCR-based microsatellite genotyping revealed a very low level of extra-pair paternity in both years and no intra-specific brood parasitism. Just three extra-pair chicks (1.0% offspring in 2.5% of broods) were discovered among 305 chicks in 112 broods. This low frequency of extra-pair paternity is significantly lower than the rates reported in three other populations of house sparrows and provides further evidence for a low level of extra-pair paternity occurring in isolated populations.;(3) The very low frequency of extra-pair paternity in this population allowed an examination of the costs and benefits that may be gained by a female exhibiting a preference for a large-badged male, unconfounded by the effects of extra-pair behaviour.;(4) The direct benefits models of sexual selection were tested by assessing male help in provisioning chicks at the nest. Counter to the predictions of these models, large-badged males contributed relatively fewer feeds than males with smaller badges. Similarly, large-badged males, and the females that chose them as maters, had lower annual fecundity and were predicted to recruit significantly less offspring into the breeding population.;5) A female preference might be driven by the indirect benefits of obtaining genes for either viability or attractiveness for the female's offspring. A cross-fostering experiment revealed that variation in badge size had a large environmental component with a strong correlation between offspring badge size and that of their foster father, with no discernible additive genetic variation. This mechanism for the determination of badge size cannot support a process of Fisherian 'runaway' selection and is consistent with those models which require a sexual ornament to be phenotypically plastic and therefore provide an honest signal.