Quantifying the impacts of stoat and cat incursions on reintroduced bird populations in a predator-fenced wildlife sanctuary

Control of introduced predators is essential for conserving many threatened species, but species range in vulnerability. Therefore, efficient conservation management requires estimating the vulnerabilities of different threatened species to introduced predators. Here, we quantify population responses of reintroduced toutouwai (Petroica longipes), popokatea (Mohoua albicilla) and tīeke (Philesturnus rufusater) to incursions of stoats (Mustela erminea) and cats (Felis catus) to a 588-ha predator-fenced sanctuary in Aotearoa New Zealand. There were fewer than 0.5 detections per year for both predator species from 2004 to 2016, but stoat detections increased >10-fold from 2017 to 2019 and cats >30-fold from 2020 to 2021. We estimated the growth and persistence of each bird population pre- and post-2017. This involved fitting integrated population models to survival, reproduction and count data for toutouwai and tīeke, and fitting a variation of the Moran–Ricker model to 5-min point counts for popokatea. We used these models to derive λ_max, the finite rate of increase at zero density, which must be >1 for a population to persist. Popokatea showed no sign of impacts, with λ_max estimated to be 1.68 (95% CRI 1.49–1.97) up to 2017 and 1.87 (1.42–2.62) after 2017. Toutouwai had tentative decreases in survival and reproduction, dropping the estimated λ_max from 1.28 (1.10–1.51) to 1.06 (0.83–1.41). Tīeke survival dropped dramatically from 2017 to 2019, but returned to pre-2017 levels when stoats were reduced, but recruitment was reduced and remained low, presumably due to cats. λ_max was estimated to be 1.74 (1.04–2.70) on pre-2017 rates; 1.14 (0.80–1.71) if only recruitment were predator-affected; and 0.79 (0.36–1.36) if both adult survival and recruitment were predator-affected. Our results therefore indicated that this level of stoat and cat incursion was inconsequential for popokatea, tentatively reduced toutouwai persistence from safe to marginal, and would have driven tīeke to extinction.