Copulation rate and sperm use by female bearded tits, Panurus biarmicus.

We examined functional and mechanistic aspects of sperm competition in the bearded tit by determining (1) the variation in copulation rate between birds breeding alone or with other males present, and (2) the number of sperm present in the female reproductive tract estimated from the number of sperm trapped on the perivitelline layers of eggs. Females copulated at a higher rate with their partner when other males were present, but this did not translate into more sperm on eggs, possibly because insemination rates exceeded the female's sperm storage capacity. The rate of sperm loss from the female's reproductive tract, which is an important variable of sperm competition models, was obtained for the first time for a wild bird and was relatively high compared with other birds. This suggests that bearded tits copulate frequently because a single insemination is insufficient to fertilize the whole clutch, and females thus have to copulate during the egg-laying period to avoid infertile eggs. This finding is the first empirical support for the fertility insurance hypothesis. In line with this we discuss the significance of the interspecific variation in rate of sperm loss in relation to mating strategies in general, and the evolution of multiple mating rather than improved sperm storage in bearded tits with particular regard to the pronounced extrapair copulation behaviour of females. Copyright 1998 The Association for the Study of Animal Behaviour.