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

Abstract

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.

Keywords: Tockus leucomelas; brood reduction; cavity nesting; hatching asynchrony; hornbills; nestling growth; nestling mortality.

FIGURE 1

Inside a southern yellow‐billed hornbill nesting cavity. Photograph depicts a female southern yellow‐billed hornbill ( Tockus leucomelas ) in a nest box with her two nestlings. Photo credit: Dirk Heinrich.

FIGURE 2

Hatch order influences nestling mass during development. Nestling mass at a given age significantly differed by hatch order during (A) the linear growth phase (Days 6–19 post‐hatching) and (B) the asymptotic growth phase (Days 20–39 post‐hatching). Shaded areas represent 95% confidence intervals.

FIGURE 3

Mass at fledging does not differ among hatch orders, likely because later‐hatched nestlings spent more time in the nest prior to fledging. (A) Nestling mass at fledging did not significantly differ based on hatch order (p > 0.99 for all post‐hoc pairwise comparisons). (B) Age at fledging significantly differed across hatch orders, with first‐hatched nestlings fledging significantly earlier than second‐hatched (t = −2.07, df = 76, p = 0.04) and third‐hatched nestlings (t = −3.93, df = 76, p < 0.01).

FIGURE 4

Maternal presence in the nest is positively associated with growth rates of younger nestlings. During the linear phase of growth, later‐hatched nestlings had significantly lower overall growth rates (robust LMM: t = −5.77, df = 111, p < 0.001). This difference was mitigated by presence of the female, as later‐hatched nestlings (i.e., hatch order 2 and 3) had relatively higher growth rates when the mother was present in the nest (t = 3.96, df = 19, p < 0.001). Error bars represent ±1 SEM. Note that there was only one instance where a female was absent from the nest during the linear growth phase of a first‐hatched nestling, and thus SEM calculation was not possible for that bar.

FIGURE 5

Path model representing causal hypotheses for the direct and indirect mechanisms of brood reduction. Arrows represent the causal direction by which variables could influence brood reduction. Values represent standardized path coefficients, and asterisks denote significant paths (*p < 0.05; **p < 0.01; ***p < 0.001).

FIGURE A1

Southern yellow‐billed hornbill nestling growth trajectories across development. The bounds of the linear (6–19 days) and asymptotic (20–39 days) growth phases are shown with dashed lines.

FIGURE A2

Impact of hatch order on brood reduction probability and age at mortality. (A) In 88.6% of known cases of brood reduction, the youngest nestling in the brood died. Such instances are represented by points that fall along the dotted line (i.e., where the hatch order of the dead nestling is equal to the size of the brood). (B) Hatch order significantly predicted the age at which brood reduction occurred (LMM: t = −6.60, df = 19, p < 0.0001), with later‐hatched nestlings tending to die at earlier ages than earlier‐hatched nestlings.