Bone stable isotopes indicate a high trophic position for New Zealand’s extinct South Island adzebill (Aptornis defossor) (Gruiformes: Aptornithidae)

The adzebills (Aptornithidae) were an ancient endemic lineage of large flightless Gruiformes that became extinct shortly after Polynesian settlement of New Zealand. The diet and ecology of these enigmatic birds has long been a matter for conjecture, but recent stable isotope analyses of bones of the North Island adzebill (Aptornis otidiformis) have indicated that adzebills may have been predatory. Here, we add to our understanding of adzebill ecology by providing the first stable isotope analyses of South Island adzebill (A. defossor) bones from two Holocene deposits. We interpret the results within frameworks of stable isotope measurements on bones of faunal species with known diets and from the same deposits (thereby mitigating regional effects on isotope values). Our results strongly support the hypothesis that adzebills had a high trophic position. Considered alongside the unique skeletal adaptations of adzebills and the distribution of the species during the Holocene, we suggest that adzebills were most likely terrestrial predators restricted to dry podocarp forests and may have specialised in dismantling rotting logs to obtain invertebrates and/or excavating burrowing animals such as tuatara or the chicks of burrow-nesting birds.