No effect of parental quality or extrapair paternity on brood sex ratio in the blue tit (Parus caeruleus)

Sex allocation theory predicts that parents should manipulate brood sex ratio in order to maximise the combined reproductive value of their progeny. Females mating with high quality males should, therefore, be expected to produce brood sex ratios biased towards sons, as male offspring would receive a relatively greater advantage from inheritance of their father's characteristics than would their female siblings. Furthermore, it has been suggested that sex allocation in chicks fathered through extrapair fertilizations should also be biased towards sons. Contrary to these predictions, we found no evidence that the distribution of sex ratios in a sample of 1483 chicks from 154 broods of blue tits (Parus caeruleus) deviated significantly from that of a binomial distribution around an even sex ratio. In addition, we found no significant effect on brood sex ratio of the individual quality of either parent as indicated by their biometrics, feather mite loads, time of breeding, or parental survival. This suggests that females in our population were either unable to manipulate offspring sex allocation or did not do so because selection pressures were not strong enough to produce a significant shift away from random sex allocation. The paternity of 986 chicks from 103 broods was determined using DNA microsatellite typing. Extrapair males sired 115 chicks (11.7%) from 41 broods (39.8%). There was no significant effect of paternity (within-pair versus extrapair) on the sex of individual offspring. We suggest that, in addition to the weakness of selection pressures, the possible mechanisms responsible for the allocation of sex may not be sufficiently accurate to control offspring sex at the level of the individual egg.