Sperm competition and unhatched eggs in the House Sparrow

There is considerable debate over the benefits that female birds obtain from engaging in extra-pair copulations (Westneat et al. 1990, Birkhead and Moller 1992). Two main classes of benefit have been identified: direct benefits, which includes fertility insurance (e.g. Sheldon 1994), and indirect or genetic benefits (see Birkhead and Moller 1992, Moller 1994). In the House Sparrow Passer domesticus, Wetton and Parkin (1991) found that the occurrence of unhatched eggs was associated with extra-pair paternity in the same clutch and suggested that this might occur because females guard against low fertility of their mate by engaging in a relatively small number of extra-pair copulations. Both Birkhead and Moller (1992: 15) and Lifjeld (1991) offered alternative explanations for Wetton and Parkin's (1991) result. Lifjeld (1994) suggested that unhatched eggs may have been the consequence of social interactions. That is, female House Sparrows paired to poor quality males would be more likely to seek extra-pair copulations, and because extra-pair copulations in this species are often forced and occur during 'communal displays' (Moller 1987), these females would be subject to an unusual amount of harassment As a consequence, not only mould these females lay a greater proportion of infertile eggs, they mould also produce some eggs fathered by extra-pair males.