JOURNAL OF AVIAN BIOLOGY 31: 237 – 244. Copenhagen 2000 Low frequency of extra-pair fertilisations in House Sparrows breeding at high density Jose ´ P. Veiga and Luis Boto Veiga, J. P. and Boto, L. 2000. Low frequency of extra-pair fertilisations in House Sparrows breeding at high density. – J. Avian Biol. 31: 237–244. 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. J. P.Veiga, Departamento de Ecologı ´a Eolutia, Museo Nacional de Ciencias Natu - rales, C.S.I.C., Jose ´ gutie ´rrez Abascal 2, 28006, Madrid, Spain. L. Boto, Laboratorio de Sistema ´tica y Eolucio ´n Molecular, Museo Nacional de Ciencias Naturales, C.S.I.C., Jose ´ Gutie ´rrez Abascal 2, 28006, Madrid, Spain. E-mail: jpeiga@ mncn.csic.es With the recent wide use of molecular techniques to ascertain paternity in birds, it has been realised that part of the offspring produced in a population results from extra-pair fertilisations (EPFs). The frequency of EPFs is highly variable between species, ranging from absence (e.g., Gyllensten et al. 1990) to more than 80% of all fertilisations (e.g., Lifjeld et al. 1993, Dunn et al. 1994a, Barber et al. 1996). There may be considerable variation between populations, but always lower than in interspecific comparisons (reviewed by Westneat and Sherman 1997, Petrie and Kempenaers 1998). While there are several hypotheses to explain the ultimate cause of interspecific variation in EPFs (Birkhead and Møller 1992, Møller and Birkhead 1994), there is no convincing explanation of the inter-population variabil- ity in those cases when it has been documented. It has been proposed that population density is one of the factors that may favour extra-pair paternity (Birkhead et al. 1987, Møller 1991). Some authors have found evidence of this (Westneat et al. 1990, Gowaty and Bridges 1991, Lifjeld et al. 1991, Gelter and Tegelstro ¨m 1992, Hasselquist et al. 1995), and have even proposed that coloniality may have evolved to facilitate EPFs (Morton et al. 1990, Hoi and Hoi-Leitner 1997). Other authors, however, have not found differences in EPF frequencies between males nesting in colonies or solitar- ily (Dunn et al. 1994b). In the Pied Flycatcher Ficedula hypoleuca, Norwegian populations breed at lower den- sities than Swedish populations, and exhibit lower fre- quencies of EPFs than the latter (Lifjeld et al. 1991, © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 31:2 (2000) 237