IN many passerine birds, sperm competition(1,2) is intense and extrapair paternity frequent(3). The outcome of competition is often determined by relative sperm numbers(4,5), theory predicts that males should maximize the number of sperm they ejaculate during extra-pair copulations(6,7). Differences In sperm quality between males also affect the outcome of sperm competition(4). Here we report that the swimming velocity of sperm of the zebra finch, Taeniopygia guttata, varies predictably within males, and is determined, together with sperm numbers, by the time since last ejaculation. By performing extra-pair copulations outside their own-pair copulation period, males maximize both the quality and number of sperm in ejaculates. These effects are a consequence of the way sperm are stored and mature in the male reproductive tract. The disportionate success of extra-pair copulations(8), also seen in other birds(9), may therefore be explained in terms of the independent effects of sperm numbers and velocity.