Nestling Growth and Brood Reduction in the Southern Yellow-Billed Hornbill (Tockus leucomelas)

Hatching asynchrony is common among altricial bird species and has long been hypothesized to facilitate brood reduction, which in turn may maximize reproductive outputs in environments with variable resource availability. Despite its prevalence, the drivers and mechanisms of brood reduction are not well understood for many species. The southern yellow-billed hornbill, Tockus leucomelas, is a useful model for understanding brood reduction because it exhibits extreme hatching asynchrony and a unique nesting strategy where the female and the brood are sealed within a cavity, sheltered from predators. Here, we documented aspects of breeding biology in artificial nest boxes and assessed the influence of various factors on nestling growth and mortality. Earlier-hatched nestlings had higher growth rates throughout development and were more likely to survive to fledge than their younger siblings. Maternal presence in the nest had positive impacts on growth rates of later-hatched nestlings, likely reflecting the female's role in mitigating sibling competition. Despite differences in growth and survival during the nesting period, weight at fledging did not differ according to hatch order, likely because later-hatched nestlings spend more time in the nest before fledging. Using a path analysis, we show that brood size and growth rates of youngest nestlings are significant direct predictors of brood reduction. In addition, our analysis suggests that rainfall (a proxy for resource availability) may indirectly influence the likelihood of brood reduction via effects on growth rate of the youngest nestling, although estimated effect sizes were small. The observed relationships between hatch order, brood size, nestling growth and mortality, and environmental variables provide support for some predictions of the brood reduction hypothesis for the function of hatching asynchrony and advance our understanding of brood reduction dynamics in this species.