Effects of a Native Avian Predator and Colony Density on Breeding Success of a Burrow-Nesting Se

• The investigation addresses native predator–prey dynamics on islands, focusing on how a native rail, the weka (Gallirallus australis), may influence breeding success of a burrow-nesting seabird, the tītī (Ardenna grisea), on the Rakiura Tītī Islands in southern New Zealand.
• The study situates itself within conservation-relevant questions about how native predation interacts with colony density and broader threats to seabird populations, particularly when colonies are large enough to potentially dilute per-capita predation pressure.

• A field-based assessment combined burrow surveillance and camera documentation across multiple sites.
• The sample comprised 126 tītī nests tracked for breeding success, distributed across three sites where weka were present and two sites where weka were absent.
• Nest success was operationally defined as the probability that an egg would hatch and the chick would fledge.
• Burrow density served as a proxy measure for colony density, enabling evaluation of density-dependent effects on breeding outcomes.
• Weka activity was monitored with trail cameras to characterize temporal patterns and potential periods of elevated predation risk.

Crucially, weka activity at monitored burrows did not appear to increase during the period of heightened nest vulnerability (mid-January to late February).

• Breeding success among tītī varied by site, with observed probabilities ranging from 0.39 to 0.88.
• Model-derived estimates indicated that the presence of weka had a negligible overall effect on tītī breeding success.
• An important positive association emerged between tītī burrow density and breeding success, although both the density effect and the predator effect carried substantial uncertainty in the estimates.
• Trail camera observations showed weka activity to be largely diurnal.

• The results support the hypothesis that weka predation on tītī may be inversely density-dependent: larger colonies may absorb predation pressure, reducing per-nest risk.
• The authors caution that the current conclusions are limited by uncertainty in the estimated effects and the observational scope.
• A potential future decline in tītī populations due to factors such as climate change or harvesting could alter the predator–prey dynamic, potentially increasing the impact of weka on breeding success and potentially accelerating localized declines if declines occur.

• The study notes substantial uncertainty surrounding both the predator effect and the density effect, underscoring the need for continued monitoring across additional sites and temporal spans.