• clutch size;
  • Cyanistes caeruleus ;
  • fertility;
  • laying order;
  • sperm depletion

Hatching failure negatively impacts reproductive success in birds. One reason why eggs fail to hatch is that they are not fertilized, which may be because they receive insufficient sperm. In most passerines, copulation declines in frequency or ceases altogether after the laying of the first egg, so eggs laid late in the laying sequence may be more likely to remain unfertilized. We tested this prediction in the Blue Tit Cyanistes caeruleus, a species in which late-laid eggs are particularly likely to be sperm-limited because it lays the largest clutch of any passerine. We assessed whether: (1) eggs laid later in the laying sequence are more likely to fail to develop, (2) larger clutches are more likely to contain undeveloped eggs, (3) the number of sperm present on the perivitelline layers of each egg decreases across the laying sequence and (4) the number of sperm present on the perivitelline layers of freshly laid eggs sampled from active clutches predicts the proportion of eggs that develop in the remainder of the clutch. The occurrence of undeveloped eggs was not related to their position in the laying sequence, nor was it related to clutch size. Within a clutch, sperm number did not differ between an egg laid early and an egg laid towards the end of the laying sequence. Moreover, there was no indication that the proportion of undeveloped eggs in a clutch correlated with the number of sperm in a subset of eggs from that clutch, or that females laying larger clutches had stored more sperm. In summary, we found no evidence that Blue Tit eggs fail to hatch because they receive insufficient sperm.