Offspring sex ratio is related to male body size in the great tit (Parus major)

Sex allocation theory predicts that the allocation of resourcesto male andfemale function should depend on potential fitnessgain realized throughinvestment in either sex. In the greattit (Parus major), amonogamous passerine bird, male resourceholdingpotential (RHP) andfertilization success both depend on malebody size (e.g., tarsus length) andplumage traits (e.g., breaststripe size). It is predicted that the proportionof sons ina brood should increase both with male body size and plumagetraits,assuming that these traits show a father—offspring correlation.Thiswas confirmed in our study: the proportion of sons in the broodincreasedsignificantly with male tarsus length and also, though notsignificantly,with the size of the breast stripe. A sex ratio bias inrelationto male tarsus length was already present in the eggs because(1) thebias was similar among broods with and without mortalitybefore the nestlings'sex was determined, and (2) the bias remainedsignificant when the proportionof sons in the clutch was conservativelyestimated, assuming that differentialmortality before sex determinationcaused the bias. The bias was still presentamong recruits.The assumption of a father—offspring correlation wasconfirmedfor tarsus length. Given that both RHP and fertilization successofmale great tits depend on body size, and size of father andoffspring iscorrelated, the sex ratio bias may be adaptive.