The biology of the stoat ( Mustela erminea ) in the National Parks of New Zealand

The gut contents of 1514 stoats from 14 collection areas (mostly forest or forest edge) were analysed. The distribution of 2643 prey items is expressed as percentage frequency of occurrence in 1250 guts containing food. In the total collection, the frequency of occurrence of the major categories of prey was: mice 19%; rats 6%;, lagomorphs 18%; opossums 10%; birds 43%; lizards 5%; insects 41%; freshwater crayfish 2%; carrion about 1–2%. The diet of stoats in relation to age, sex, season, and habitat was examined more closely in the subsamples from forest (n = 866; three classes of forest recognised) and from Mount Cook (n = 146, the only large non‐forest sample). There were no general and consistent differences with age. In all seasons female stoats ate more mice and ground wetas, and males more opossums and ‘unidentified mammals’ (mostly opossums and rats). Grouping the prey as large, small, or medium showed that females ate more small prey ( P < 0.01) and males more large prey (0.02 < P < 0.05), but this may reflect the greater food requirements of males rather than their ability to kill larger prey; lagomorphs and rats were eaten as often by females as by males, and opossums, the only other large prey, were probably often eaten as carrion. There were differences in diet between forest types and seasons, of which at least some must have reflected real changes in availability of prey. Stoats caught at rubbish tips or around settlements had not eaten more rats and mice than stoats caught in adjacent forest. Large prey items supply most of the biomass of the stoat’s diet; insects, though frequently taken, contributed only about 10% of the volume eaten. The guts of 30 weasels sympatric with the stoats in several areas contained mostly mice, birds, lizards, and insects. There is no evidence that stoats in the New Zealand National Parks compensate for the lack of voles by eating more birds than they do in British farmland, but they do eat more insects.