Counter‐factual simulation highlights requirements for additional management to avert extinction of a relict Tītī/Sooty Shearwater population

Seabirds play key ecological roles but face increasing threats worldwide. Among them, Tītī (Sooty Shearwater) Ardenna grisea are among the most abundant and culturally significant species in the Southern Hemisphere but are also threatened. Kapiti Island, Aotearoa (New Zealand), holds a Tītī colony which, despite eradication of non‐native mammalian predators, continues to decline. Key drivers are changing oceanic conditions and depredation of chicks by Weka Gallirallus australis (a native rail). A neighbouring island, Mana Island, also holds a Tītī population, but no Weka, offering a natural ‘control’ site. Despite the decline of Tītī on Kapiti, and wider Aotearoa, and their ecological and cultural importance, few comprehensive estimates of vital rates exist (i.e. synthesized from multiple disparate data streams to mitigate uncertainty). We fitted data collected from Tītī at both Mana (1991–2025) and Kapiti (2022–2025) Islands to an integrated population model (IPM) to estimate vital rates and population size and to project the Kapiti population under two scenarios: status quo and a counterfactual ‘ no Weka ’ scenario informed by productivity estimates from Mana. Adult annual survival was estimated at 0.900 (95% CI 0.875–0.920), immature (i.e. prior returning to the colony as an adult, which peaked at ages 6–7) survival at 0.776 (0.735–0.816), breeding probability at 0.807 (0.723–0.879), breeding success on Kapiti (Weka) at 0.019 (0.005–0.043), breeding success on Mana (no Weka) at 0.420 (0.374–0.468) and adult population size on Kapiti at 237 (147–313). Without effective intervention, projections from 2025 predict a 70% decline in the Kapiti population by 2050 ( = 0.953; 0.848–1.00). However, counterfactual projections still show ongoing, albeit slower, declines ( = 0.969; 0.875–1.000), suggesting other pressures in addition to Weka predation. Our findings support future decision‐making for conservation management on Kapiti, highlight the need for action beyond the island to address external stressors, and bear relevance to seabird and Weka management throughout Aotearoa to protect the iconic Tītī.