When planning conservation translocations of species from ex-situ environments to the wild, it is essential to ensure that captive populations can act as reservoirs for bolstering or replacing wild populations without damaging their own viability. However, many ex-situ populations of highly threatened species suffer from low reproductive success, with low genetic diversity hypothesised to be a critical constraint on pre-zygotic and pre-hatching survival rates, and thus on population growth. Here, we interrogate the factors that influence these rates in the sihek (Guam kingfisher, Todiramphus cinnamominus), an extinct-in-the-wild species that has conservation translocations to the wild planned imminently. The sihek population suffers from high inbreeding load, with dam inbreeding coefficient negatively correlated with reproductive success. Population trajectories indicate that specific management strategies are required to ensure the sihek population’s persistence, presenting a need to understand which aspects of the reproductive process influence low success. We find extremely low hatching success (20%) and egg development rates (47%) in the sihek population, compared to other threatened bird species. Using Bayesian generalised linear mixed models, we investigate the effect of parental factors (e.g., inbreeding, age) and incubation strategy on both rates in the sihek using records of 539 eggs laid in captivity over the past 35 years. Our models demonstrate that parental effects and incubation type do not influence egg development or hatching success rate. Therefore, we predict that growing the sihek population for the purpose of harvesting individuals for translocation will not be hindered by high levels of inbreeding at pre-hatching development stages.