Authors: Morland, Fay; Hemmings, Nicola;Brekke, Patricia ;

Monitoring reproductive success is an important part of species recovery and reproductive failure can hinder conservation efforts. In birds, small, threatened populations often experience high rates of egg hatching failure, which can be due to either fertilisation failure or embryo mortality. The mechanisms behind these two causes of hatching failure differ, as it is likely that the factors influencing them do. However, distinguishing between fertilisation failure and embryo mortality is not commonplace in the monitoring of bird populations. Using a dataset of over 1,000 failed eggs, laid by 187 females across 8 years, we examine long-term patterns of infertility and embryo mortality, and their association with population demographic factors in a reintroduced, managed population of a threatened bird species, hihi (Notiomystis cincta). We find that although average hatching failure rates are stable over time, fertilisation rates vary across years. Variation in egg fertilisation rates is significantly related to population size and sex ratio, which are intrinsically linked; with more females resulting in a larger overall population size. We find that birds lay more unfertilised eggs in years when the population is smaller and the sex ratio more male biased. We discuss these results in the context of conservation management.