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 extr a-pair fertilisations (EPFs) is controversial. Some evidence supports the i dea that population density and frequency of extra-pair paternity are posit ively associated, whereas other work does not. In the present paper we esti mate EPF frequency in a dense House Sparrow Passer domesticus colony. We de tected extra-pair nestlings in 9.3% of 54 broods studied, and 7% of 171 nes tlings were sired by extra-pair fathers. The number of clutches laid per fe male, the change of male or female between two consecutive breeding attempt s and the age of the partners showed no association with the presence or ab sence of extra-pair fertilisations. Morphometric variables of paired males and females did not discriminate broods with EPFs from those without. We de tected 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 br eeding at lower densities, or other species that breed in colonies. This re sult does not support the claim that EPF Frequency is associated with popul ation density. We propose, as an explanation for this result, that under hi gh intra-sexual competition for nest sites (1) males may have limited oppor tunities to search for females for extra-pair copulations and (2) the high quality of male nest-owners may reduce Female propensity to search for addi tional sexual partners.