The effectiveness of predator-proofing nest sites to support the breeding of an endangered parrot

Invasive predators present a major threat to the conservation of native species. For birds, this often comes in the form of increased nest predation. Conservation managers have used numerous interventions to reduce this risk. However, their effectiveness varies depending on the specific technique used and the predators targeted. In this study, we tested the effectiveness of nest site modifications used to protect endangered Norfolk Island Green Parrot Cyanoramphus cookii nests from introduced mammalian predators. We compared the breeding outcomes and nest site characteristics of a sample of natural, unprotected nests with those of predator-proofed nests, and examined contemporary phenology and breeding parameters. We observed moderate, but variable, breeding seasonality, and significantly fewer nesting attempts in recent years. Nests had a median clutch size of five (range = 1–10) and a median brood size of three (range = 1–7). Nest success was 74.8% (157/210) for predator-proofed nesting attempts, and 40% (4/10) for unprotected nesting attempts. Predator-proofing had a small but significant effect on daily survival rate (estimated at 0.995 for predator-proofed nests and 0.972 for unprotected nests), but did not significantly affect the number of birds fledged. We observed nest predation by both Black Rats and feral Cats at unprotected nests. Unprotected nest sites had significantly shorter chambers than predator-proofed nest sites, suggesting that while predator-proofing significantly increases nest success for Green Parrots, preferred nest site characteristics have not been preserved through modifications. These findings highlight the importance of assessing the effectiveness of management actions regularly to inform evidence-based decision-making and adaptive management.