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.

Many bird species show delayed plumage maturation (DPM), retaining sub-adult plumage until after their first breeding season. Most explanations assume that DPM increases fitness over the breeding season. However, unless birds undergo a full moult before breeding, DPM could also be an adaptation to increase survival over the previous winter. The winter adaptation hypothesis has never been tested owing to the difficulty of measuring overwinter survival. We experimentally tested this hypothesis in North Island robins (Petroica longipes) using a closed island population where we could accurately estimate survival. The experiment involved dyeing 41 juveniles to mimic adult males, and comparing their survival with 41 control juveniles treated with the same peroxide base minus the pigment. The population was monitored with a series of resighting surveys, and mark-recapture analysis used to estimate overwinter survival. Survival probability was estimated to be 10% for dyed birds versus 61% for control birds in 2001, and 29% for dyed birds versus 40% for control birds in the winter of 2002, supporting the winter adaptation hypothesis for DPM. Access to suitable habitat is the key factor limiting juvenile survival in this population, and the locations where dyed juveniles were sighted suggest that they were often excluded from suitable areas.

Wildlife managers often have a good understanding of the threats faced by populations, but they need to know the intensity of management required for populations to survive. Managers therefore need quantitative projections for populations under different management regimes rather than just qualitative comparisons. However, quantitative projections are subject to tremendous uncertainty, particularly for small populations monitored for short time spans. We assess the level of predator control needed for a reintroduced population of North Island robins (Petroica longipes) to grow, accounting for uncertainty associated with parameter estimation, model structure, and demographic stochasticity. The robin population grew when exotic rats were reduced to low levels (<10% of footprint tunnels tracked in 24 hr) by regular maintenance of poison bait stations. However, the population declined after baiting was stopped 3 years after the reintroduction (March 2002), and it had fallen to 4 pairs by September 2004. We created a simulation model incorporating relationships between vital rates (survival and fecundity) of the robin population and rat tracking rate estimated from 5 years of data. We ran the model 10,000 times at each rat tracking rate, with vital rates sampled from distributions (defined by estimates and standard errors) at the start of each run. Output from a deterministic model suggested that λ (finite rate of increase) would be >1 if rat tracking were <20%, and up to 1.2 with rat tracking at 1%. However, 95% confidence intervals for λ extended <1 at any tracking rate. With demographic stochasticity added, there was >20% probability of further decline in 5 years even when the expected λ was 1.2. With all forms of uncertainty included, 41% of simulations projected a further decline over 5 years if the rat tracking rate were 0%. This proportion was reduced to 30% if initial population size was increased to 20 pairs. Our analysis therefore showed it was most likely that the robin population would grow if intensive rat control were reinstated, particularly if the population was supplemented, but there was substantial risk the population would continue to decline under such management.