Sperm numbers on the perivitelline layers of blue tit eggs are repeatable within a clutch, but independent of the occurrence of extra-pair paternity

In many socially monogamous bird species, females produce offspring sired by males other than their social partner. A large body of research has aimed to elucidate the evolutionary causes and consequences of such extra-pair paternity, but relatively little is known about the underlying behaviour. The number of sperm on the egg's perivitelline layers (PVL) is related to recent copulation activity and may thus give some insight into the female's mating behaviour. We used a simple technique that allowed us to remove embryonic cells from the blastoderm for DNA extraction whilst keeping the PVL intact for sperm counts. Using 243 eggs from 99 blue tits Cyanistes caeruleus, we show that PVL sperm numbers were repeatable within clutches (r = 0.40 [95% CI: 0.25–0.53]). However, neither overall sperm numbers nor changes in sperm numbers across the laying sequence differed between clutches that contained extra-pair sired eggs and those that did not. Our results, therefore, provide no evidence that females with and without extra-pair young differ in their copulation activity.