Observation of fantail (Rhipidura fuliginosa) nesting success in response to Predator Free Wellington

Predation by introduced predators is a major driving force for bird declines globally, and particularly in Aotearoa New Zealand where the native fauna evolved in a system lacking mammalian predators. With high levels of endemism in New Zealand’s birds, conservation groups throughout the country are working to prevent further species loss by controlling possums, mustelids, and rats. Moreover, New Zealand has adopted the ambitious goal of eradicating these three predatory mammal groups by 2050. Eradication on such a scale has never been achieved, especially in highly urbanised environments, meaning outcomes for native species are not completely understood. One of the leading groups in this eradication is Predator Free Wellington, which successfully eliminated all target predators from Miramar Peninsula in 2023.

I analysed the effect of Predator Free Wellington’s predator elimination on the nesting success of a native forest bird, the fantail (Rhipidura fuliginosa, pīwakawaka). Independent monitoring has shown that fantail numbers have increased by 550% across the peninsula concurrently with the elimination project, hence I conducted nest monitoring through the 2023/24 breeding season to compare success rates and possible causes of nesting failure on Miramar Peninsula relative to the rest of Wellington City where elimination of predators has not been achieved.

I found that nesting success for fantails across Wellington has approximately doubled to 88% from 2015 to 2023 throughout the city, inclusive of the predator elimination zone on Miramar Peninsula as well as other areas across Wellington where predators have not been eliminated. Despite the presence of rats outside of Miramar Peninsula, I recorded no nest predation events throughout my study. The results suggest that complete eradication is not essential to increase nesting success for fantails, and existing predator control across Wellington has been sufficient to achieve this outcome. Additionally, I created an ethogram of nesting behaviours to identify stage specific behaviours that can reveal nest stages to observers without the need for invasive monitoring techniques.

The results suggest the need for research focusing on adult and immature fantail survival outside of the nesting phase to understand more fully how predator elimination affects the population dynamics of the species. Further monitoring is also needed on Miramar Peninsula following the elimination of predators in 2023 to determine how fantails will respond to the wider changes in the resultant bird and predator communities.