Aspects of sperm transport in the oviducts of female zebra finches were examined by recording the decline in the number of spermatozoa on the outer perivitelline layer of successively laid eggs. Data from a single clutch of eggs from 32 females indicates that the mean per capita rate of loss of spermatozoa was estimated to be 0.0170 +/- 0.002 SEM spermatozoa h-1. However, individual females showed no consistency in the rate at which spermatozoa were lost from their oviduct over five successive clutches. Models of the mechanism of sperm competition in birds assume that the rate of loss of spermatozoa does not differ between inseminations made before or after the start of egg laying. This assumption was found to be valid: the instantaneous per capita rate of loss of spermatozoa did not differ significantly between females inseminated either before (0.01445 +/- 0.0028 SEM spermatozoa h-1) or after (0.01674 +/- 0.0023 SEM spermatozoa h-1) the onset of oviposition. The rate of sperm transport through the infundibulum was determined to be slower than that between the utero-vaginal sperm storage tubules and the infundibulum by comparing the number of spermatozoa associated with the perivitelline layers of eggs laid after a day on which no eggs were laid. Eggs with < 20 spermatozoa on the outer perivitelline layer were found to have a 50% probability of being infertile. The results are compared with data for domestic fowl and turkeys.