Modeling Vital Rates of a Reintroduced New Zealand Robin Population as a Function of Predator Control



Abstract The introduction of rats and other mammalian predators has caused many New Zealand species to decline. Predator control is now being used to reverse these declines in selected mainland areas, and a footprint-tracking index is used to assess effectiveness of control. To assess the meaning of this index for native populations, it is necessary to model the functional relationships between predator-tracking rates and vital rates of native populations. We monitored North Island robins (Petroica longipes) for 5 years after reintroduction to Paengaroa Mainland Island, and rat levels changed dramatically over this period due to changes in management policy. We used the resulting data to model how vital rates varied with rat tracking, using Akaike's Information Criterion to compare alternative models for each vital rate. We fitted survival models to mark-resighting data obtained in tri-annual surveys of the reserve, and we fitted fecundity models to data on numbers of independent young produced by individual females. The best model for annual adult survival was š = 0.64p0.24F where p is the complement of the tracking rate (i.e., 0 = 100% tracking) and F is sex (0 = male, 1 = female). The best model for annual fecundity per female was where U is the female's pairing status (0 = paired, 1 = unpaired). For juvenile survival (from independence to adulthood), it was ambiguous whether survival was constant (š = 0.39) or changed with rat levels (š = 0.49p0.58). We used the delta method to obtain a 95% confidence interval for each vital rate at any rat tracking rate. The models allow the growth of the population to be projected at any tracking rate, and they provide a starting point for projecting growth of any population in relation to a predator-control index.