The density of a ferret population in South Island pastoral farmland was estimated by live capture, mark and release, and by removal trapping, during March and April 1997. An estimate of 6.3 ferrets per km 2 (5.5-8.4, 95% CI) was obtained after 8 days of mark and release, whereas 10 days of removal trapping provided an estimate of 5.3 ferrets per km 2 (5.3-5.9, 95% CI). There was significant variation in individual trappability. Proportionally more unmarked ferrets were captured as population density was reduced during removal trapping. Four person-days effort were required to remove 50% of the ferret population from an area of 1254 ha, while removal of 80 or 95% of the population took considerably greater effort. Trapping provided a reliable relative index of ferret abundance. While there is a need to replicate this study in order to confirm its validity, it confirms the use of a relative index as a valuable tool for measuring the efficacy of control operations and estimating ferret abundance.
Breeding colonies of sooty shearwaters ('muttonbird', tïtï, Puffinus griseus) on mainland New Zealand have declined in recent years. New data on burrow occupancy and colony productivity for seven sooty shearwater breeding colonies on the coast of Otago, New Zealand for the 1996–97 and 1997–98 breeding seasons are presented and analysed as part of a five-year data set. Detection of a burrow's occupants using a fibre-optic burrowscope may underestimate absolute occupancy rates, but is still of value in the analysis of trends. Detection probabilities estimated by the novel use of mark–recapture models corresponded with those of previous studies of the technique's accuracy. Mainland declines are associated with a lack of control of introduced mammalian predators at most mainland colonies superimposed on a global pattern of decline in the species' abundance. Large numbers of recovered carcasses and an absence of burrow activity at two small mainland colonies show the decline to extinction of these colonies over the five years of collecting data. At one mainland colony with intensive predator control, survival rates and parameter variances are comparable with those found on a predator-free offshore island. All other mainland colonies showed negligible breeding success. There was a significant positive relationship between egg survival and an index of relative adult survival, with an apparent threshold below which few eggs hatch. Adult survival during the breeding season is likely to be the most important parameter in maintaining a colony's viability.
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