| Summary: | This study examined the breeding ecology of the house sparrow, Passer domesticus, in Kentucky, USA, with particular regard to sexual selection, infidelity and parasitism. These aspects were also examined, to a lesser extent, in an archipelago population in Helgeland, Norway.;Male badge size, a character posited to be under sexual selection, did not appear to influence reproductive success. Large-badged males produced more fledglings within a season than small-badged males, although this was not significant after controlling for time of breeding. Large-badged males did not commence breeding earlier than small-badged males, they were not paired to higher quality, more fecund females, and their young did not fledge in better condition. Badge size was not related to an individual's age or condition, and although badges varied in their degree of asymmetry, this was not related to any measures of reproductive success. Badge size did not influence reproductive success in Helgeland.;The level of extra-pair in both populations was relatively low [10.3% of young in Kentucky, 4% of young in Helgeland (based on retrospective identification of parents)]. No extra-pair fathers were assigned, although there were no obvious pheotypic differences between males which were cuckolded and those with complete paternity within their broods. There was no association between cuckoldry and either infertility, breeding synchrony or density. Males appeared to rely upon frequent copulation as opposed to mate guarding as their main means of paternity protection. Copulation rates were unrelated to male sperm reserves as measured by the size of their cloacal proturberance.;Females did not adjust the sex ratio of their brood in response to their own physical condition or the attractiveness or quality of their mate.;Hatching asynchrony and brood reduction were both common in Kentucky, although the two phenomena were not associated.
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