Post-hatching testosterone concentration reflects nestling survival and pre-fledging offspring condition in the Tawny Owl Strix aluco

We examined parental condition, brood sex composition, hatching order and post-hatching testosterone concentration in relation to nestling survival and pre-fledging offspring condition on Tawny Owls Strix aluco in Duna-Ipoly National Park, Hungary in 1996– 2007. In smaller broods, where parents of poor condition suffered nestling mortality, hatchlings showed a male-biased sex ratio, and between-sibling differences in testosterone concentrations were high in the early nestling period. In broods where all nestlings were raised and parents were in good condition, hatchlings showed a female-biased sex ratio and between-sibling differences in testosterone concentrations were low. First- and second-hatched nestlings, with high post-hatching testosterone concentrations, survived after one or two siblings died and had better body condition before fledging than nestlings which fledged from the broods, where all were chicks raised.