The insemination window provides a distorted view of sperm competition in birds

The aim of this study is to identify and rectify a misunderstanding about the optimal timing of inseminations in birds. In species laying clutches of more than one egg, a copulation during the hour following egg-laying can result in sperm reaching the site of fertilization in time to fertilize the next egg to be laid. Cheng et al. (1983) referred to this period as the insemination window and proposed that it was 'an especially favourable period' for males to obtain extra-pair copulations. As stated in their paper, this is true only in terms of the next ovum to be fertilized, but subsequent authors assumed that the insemination window represents a general peak in female fertility and have made predictions about the optimal timing of extra-pair behaviours and paternity guards relative to it. Far from being a general peak in female fertility, we show by a re-analysis of Cheng et al.'s data and by using published information on the domestic fowl Gallus domesticus, turkey Gallopavo meleagris and Muscovy duck Cairina moschata, that inseminations either just after egg-laying or just before it are much less likely overall to result in fertilization than inseminations made at other times. The reduced efficacy of inseminations made close to the time of egg-laying occurs because the retention of sperm by females inseminated at this time is low. The fact that inseminations made around the time of egg-laying in the domestic fowl, turkey and Muscovy duck have a reduced probability of fertilization is consistent with the fact that very few wild birds, even those in which sperm competition is intense, alter their copulation or mate guarding behaviour during the insemination window.