The effects of beech masts and 1080 pest control on South Island robins (Petroica australis)

We assessed the effect of aerial 1080 control of possums (Trichosurus vulpecula), ship rats (Rattus rattus) and stoats (Mustela erminea) on the survival and nest success of South Island robins (Petroica australis) at Tennyson Inlet, Marlborough Sounds, from 2012–2017. Cereal baits containing 1080 were applied in 2013 when rat and stoat numbers were low, and again in 2014 after a beech mast when rat numbers were high. Survival rates of 134 banded adult South Island (SI) robins and 209 SI robin nests were monitored. Ship rats were the main predator of eggs and chicks, with stoats, possums and ruru (Ninox novaeseelandiae) also preying on nests. Rats were the only predator identified killing adult females on the nest. Nest success was negatively affected by rat abundance and increased when 1080 reduced rat abundance. Our study did not detect a measurable negative effect of aerial 1080 use on nest success or adult survival, although over the course of the study two nests failed and two adults disappeared while 1080 was present in the forest. The positive effect of aerial 1080 pest control was short-lived, with rat numbers increasing rapidly in subsequent years. To achieve long-term population benefits in small beech forest blocks, future pest control operations will need to achieve higher rat kills in beech mast years, and/or be undertaken at roughly three yearly intervals.