Low frequency of extra-pair fertilisations in House Sparrows breeding at high density

The importance that the density of breeders has on the opportunity for extra-pair fertilisations (EPFs) is controversial. Some evidence supports the idea that population density and frequency of extra-pair paternity are positively associated, whereas other work does not. In the present paper we estimate EPF frequency in a dense House Sparrow Passer domesticus colony. We detected extra-pair nestlings in 9.3% of 54 broods studied, and 7% of 171 nestlings were sired by extra-pair fathers. The number of clutches laid per female, the change of male or female between two consecutive breeding attempts and the age of the partners showed no association with the presence or absence of extra-pair fertilisations. Morphometric variables of paired males and females did not discriminate broods with EPFs from those without. We detected a single case of a female laying a parasitic egg in the nest of a male that in a previous breeding attempt was the extra-pair genetic father of her entire brood. The frequency of extra-pair fertilisation recorded in this study was low compared with that in other House Sparrow populations breeding at lower densities, or other species that breed in colonies. This result does not support the claim that EPF Frequency is associated with population density. We propose, as an explanation for this result, that under high intra-sexual competition for nest sites (1) males may have limited opportunities to search for females for extra-pair copulations and (2) the high quality of male nest-owners may reduce Female propensity to search for additional sexual partners.