Brood reduction is known as an important bet hedging where avian parents can reduce the brood to maximize the number of offspring they can support, thus maximizing parental fitness according to the availability of food resources. Despite its prevalence in a large diversity of avian species, the exact mechanisms that drive brood reduction are largely unknown. In this study, we examined two competing hypotheses to determine if brood reduction occurs via sibling competition or unequal food allocation by the mother. Data was collected from wild southern yellow-billed hornbills (Tockus leucomelas) in artificial nest boxes in Namibia, Africa, and different linear models were used to quantify the importance of factors such as bird body condition, female presence, chick rank, and amount of rainfall. We found that the female is present in 73.7 % of nests that were confirmed to undergo brood reduction (n = 19). Additionally, chick rank and female presence were both important factors in chick growth, suggesting that the female favors chicks that hatched first. Sibling competition is apparent within the nest, especially after the female leaves, but the female's unequal food allocation gives older chicks an additional advantage over their siblings.
