Determinants of hatching and recruitment success for captively reared kakī (Himantopus novaezelandiae)

Abstract

Captive-rearing of wildlife for release has been used with variable success in the conservation management of a range of species. These programmes protect individuals through a vulnerable life stage with the aim of releasing them to re-enforce wild populations once threats are minimised. To maximise the effectiveness of captive-rearing, species' managers must understand how management decisions and procedures affect individual outcomes during both the rearing phase and post-release. We used management records for 1177 kakī (Himantopus novaezelandiae; black stilt) eggs and 846 released individuals collected from 2013 to 2020 to investigate: (1) effects of parentage, clutch characteristics, and embryo age on hatchability; and (2) impacts of release variables, captive-rearing conditions, supplementary feeding, and individual health on post-release survival. Multivariate generalised additive models were created to explore these relationships. Top models showed that, in general, highest hatchability was associated with eggs that were heavier, from intermediatesized clutches, with longer parental incubation, and that were laid by dams 12 to 18 years of age. We show that intensive egg pulling from nests does not have a negative impact on the hatchability of subsequent clutches (up to three). While it is important to maximise hatchability outcomes where possible, hatchability rates for the period are high and comparatively larger gains for the species can be made addressing low survival of released individuals. Trends in survivability show that individuals released as sub-adults, that used supplementary food more often, and that were less inbred, had the best survival outcomes. Having had (but recovered from) encephalitis and/or pododermatitis in captivity reduced an individual's probability to survive once released. These trends can be used to inform best practice species management and provide rationale for further study of kakī hatchability and survivability.