Data from: Intra-ejaculate sperm selection in female zebra finches

Amongst internal fertilisers, typically fewer than 1% sperm survive the journey through the oviduct. Several studies suggest that the sperm reaching the ovum – the “fertilising set” – comprise a non-random subpopulation, but the characteristics of this group remain unclear. We tested whether oviductal selection in birds results in a morphologically distinct subset of sperm, by exploiting the fact that the fertilising set are trapped by the perivitelline layer of the ovum. We show that these sperm have remarkably low morphological variation, as well as smaller head size and greater tail length, compared to those inseminated. Our study shows that the morphological composition of sperm – rather than length alone – influences success in reaching the ovum.