A variety of behaviours adopted by males before and after copulation serve to increase paternity. The most spectacular examples occur in insects where males increase their own chances of fertilizing the female's eggs by mate guarding, mating plugs and even removal of other males' sperm1,2. Here, sperm competition is described for a small European passerine bird, the dunnock (Prunella modularis), where females are often mated simultaneously to two males3,4 and where there is an elaborate pre-copulatory display5,6. It is shown that, during this display, the male stimulates the female to eject sperm before he himself copulates. The display is most intense where there is a high probability that another male has recently mated with the female.