Estimating impacts of poison operations using mark-recapture analysis: hihi (Notiomystis cincta) on Mokoia Island

Hihi (Notiomystis cincta) were reintroduced to Mokoia Island, Lake Rotorua, New Zealand, in September 1994, and two years later there was an aerial dr op of brodifacoum cereal pellets aimed to eradicate mice (Mus musculus). Us ing Program MARK, we analyzed data from resighting surveys to assess whethe r hihi had lower than normal survival in the 6-week interval following the drop. The resighting data were collected on a regular basis over a 3-year p eriod, from 1994-97, allowing us to control for yearly and seasonal variati on in resighting and survival probabilities. We initially established that the Cormack-Jolly-Seber model had a good fit to the data and could therefor e be used as the global model for our analysis. We then compared a range of simpler candidate models, some of which included a poison effect (an unusu al survival rate for the interval after the poison drop). Under the best mo del (that with the lowest AIC), the survival probability was constant over time and there was no poison effect. The estimated survival probability for the 6 weeks after the poison drop was 0.95, which is slightly higher than the value of 0.89 expected based on pre- and post-poison intervals. The app roximate 95% confidence interval for the probability of a bird dying due to poison ranged from -0.17 (i.e., a decrease in mortality rate due to the po ison) to +0.04. We therefore concluded that the poison caused at most a neg ligible increase in mortality, and that mark-recapture analysis on resighti ng data provided a powerful method for assessing the impact of the poison d rop. We discuss the relative costs and benefits of radio tagging versus res ighting surveys of banded birds for estimating impacts of poison operations . For species with relatively high resighting rates, such as hihi, analysis of resighting surveys is a much more reliable and cost-efficient methodolo gy.