Translocation to a mainland fenced sanctuary and conventional pest control: Implications for a remnant South Island robin (Petroica australis) population limited by introduced predators

Abstract

The majority of New Zealand’s native forest bird species are currently limited by introduced mammalian species, and 24% of all terrestrial bird species are ranked as threatened. While there have been some spectacular achievements in the recovery of some of the most critically endangered bird species, many forest birds continue to decline. Without intervention, many of these species are on the pathway to extinction. In this thesis I used the South Island robin (Petroica australis) as a model to investigate aspects of two important management practices to protect native birds from introduced predators: (a) translocation of native birds to sanctuaries and (b) sustained control of introduced predators.

(a) I examined the demographic and environmental drivers for the successful establishment of translocated SI robins in a fenced mainland sanctuary (Orokonui Ecosanctuary, Dunedin) where introduced predators have been eradicated. One of the main causes for reintroduction failures is philopatric post-release dispersal from the area. I tested my hypothesis that pre-dispersal robin fledglings are less likely to return to their capture sites than older birds: Robins from two different age classes, post-natal dispersal juveniles (≥ 3 month than fledging) (n=25) and pre-natal dispersal fledglings (≤2 month since fledging) (n=20), were translocated during the 09/10 and 10/11 breeding seasons, respectively. Almost a third (32%) of the post-natal dispersal juveniles returned back to their site of origin but none of the pre-natal dispersal fledglings did. The retention rate for post-natal dispersal juveniles was only 24%, while 70% of the pre-natal dispersal fledglings settled in the release area. The results indicate that choosing fledglings drastically increases the likelihood to establish a persistent founder population. In addition, I fitted all robins released in 10/11 with radio transmitters and monitored their survival and movements for six weeks. The key findings of this telemetry study were: (1) movement activity was highest immediately after release, accompanied with higher mortality, and decreased over time; (2) birds went through two distinct phases: the exploratory and establishment phase, but settlement patterns were irregular and differed from individual to individual; and (3) dispersal distances were not affected by sex, source population or release site, but there was some indication that conspecific attraction, in the form of encountering suitable breeding partners, may influence the post-release dispersal and settlement patterns of translocated robins.

(b) I investigated the effects on an established robin population of a conventional pest control operation using aerially applied cereal bait containing the pesticide 1080 in exotic plantation forest in Silver Peaks, Dunedin. Monitoring of robin nest predators, using chew-tracking-card devices, following the pest control operation showed a significant reduction of rat and possum numbers to undetectable levels for at least three months, while in the non-treatment Silverstream area, pest numbers remained high throughout the study. All individually colour-banded robins monitored in treatment (n = 19) and non-treatment (n = 15) study areas were re-sighted, indicating that no birds died as a result of the poison drop. Nesting success, measured as Daily Survival Rate (DSR), in the treatment area was significantly higher than in the non-treatment area, but was not significantly different to the previous breeding season. My study showed that the 1080 operation had no direct adverse effect on the robin population in the Silver Peaks while reducing potential nest predators to extremely low levels.

The results from this thesis contribute directly to the conservation management of South Island robins, and potentially other native territorial passerines in New Zealand, and internationally where similar conservation strategies are used. The advancements in knowledge will directly contribute to improving the success rate of conservation efforts such as translocation and predator control, and emphasise the benefits of post-release monitoring and adaptive management.