Pair bond status is influenced by male incubation effort in a translocated population of North Island brown kiwi

Long-lived bird species often form socially monogamous pairs that endure across multiple breeding seasons, and the establishment and maintenance of such long-term bonds can provide reproductive benefits, thereby enhancing overall success. Here, we used long-term monitoring data from a translocated, high-density population of North Island brown kiwi, Apteryx mantelli , to describe patterns of divorce and partner fidelity. This flightless species breeds primarily as long-term socially monogamous pairs and secondarily as nonkin polygamous units, both with male-only incubation. Some of these pairs and groups divorce, and we used Bayesian statistical models to investigate whether reproductive, social and mate quality factors influenced the occurrence of divorce and to explore the relationship between pair familiarity and reproductive success. We also explored the benefits of forming polyandrous breeding units by testing for differences in daily incubation effort between males sharing incubation in breeding groups and males incubating alone in pairs. We found a low average annual divorce rate in this brown kiwi population. Fidelity probability increased with increasing incubation effort, indicating that the more days a male incubated, the higher the likelihood of staying together. Polygamous breeding units consisted of single or multiple males that either helped (shared incubation) or did not help incubate (single incubation). Shared incubators tended to invest less in nest attendance than single incubators, suggesting benefits linked to nest relief. We suggest that mating flexibility expressed as divorce represents a secondary strategy to compensate for suboptimal partnerships and boost reproductive output. The interannual variability in incubation investment provides a fruitful field for investigating external covariates of divorce such as demographic trends and environmental factors. Unlike most studies on divorce and mating systems, we provide rare insights into the social and reproductive behaviours of a bird with cryptic reproductive habits. Our findings have potential implications for kiwi conservation. • The North Island brown kiwi, Apteryx mantelli , has a low annual divorce rate. • We found that fidelity probability increased with increasing incubation effort. • Shared incubation reduced individual nest attendance providing nest relief benefits. • Postdrought divorce rates were high as mortality left singles free to re-pair. • Divorce may compensate for suboptimal partnerships and boost reproductive output.