Parental Age, Inbreeding and Incubation Method Influence Extremely Low Hatching Success in the Ex‐Situ Population of the Extinct in the Wild Sihek

ABSTRACT Low reproductive success rates can be a common problem for threatened species, impacting population viability and limiting recovery potential. Reproductive success can also be particularly low in populations that are managed ex‐situ. This can pose a challenge for threatened species recovery programmes because high reproductive success rates are often required to provide offspring for wild releases while ensuring population viability. Understanding the underlying causes of low reproductive success rates in ex‐situ populations of threatened species is therefore essential so that management programmes can optimise species' recovery potential. Here, we quantify rates of egg viability (i.e., indicating fertility and/or early embryo mortality) and total hatching success in the Extinct in the Wild sihek (Guam kingfisher, Todiramphus cinnamominus ). Using Bayesian generalised linear mixed models, we investigate effects of parental age, parental inbreeding coefficient ( f ), egg f and incubation method on egg viability and hatching success of viable eggs. We find that the sihek population has extremely low egg viability rates (~48% ± 2.15%, N = 304/635) and total egg‐hatching success rates (~30% ± 1.82% SE 190/635) compared to other threatened and non‐threatened bird species, both ex‐situ and in the wild. We find that increased paternal age and f are key drivers of decreased egg viability. In contrast, increased maternal age and use of artificial incubation are important contributors to decreased egg‐hatching success. Our results are particularly pertinent given current active recovery planning for sihek, which may require increased offspring production for wild releases. Furthermore, our results suggest that, in closed ex‐situ populations where f inevitably increases across generations such that at a given time point older aged individuals may have lower f , there is a need for breeding recommendations to quantitively and systematically balance genetic considerations with species' biological limitations such as reproductive senescence to meet recovery programme goals.