Large numbers of stoats (Mustela erminea) were brought from Britain in the 1870s to control ‘verminous rabbits’. They immediately spread to the bush, where they preyed on native animals. Stoats are energetic, bold and versatile hunters, foraging in every hole, under any cover and up the tallest trees. They are also good swimmers.
By 1910, many native birds had disappeared. Together with rats and cats, stoats have contributed to the extinction of huia, bush wrens, native thrushes, laughing owls and quails. They also exterminated stitchbirds, saddlebacks, kākāpō and little spotted kiwi from the mainland.
Today, stoats live from North Cape to Bluff, on mountains, in farmland, scrub and bush. In the bush, they eat more birds than any other food. Rats and mice, rabbits, possums, hares, lizards and insects form the rest of their diet. In one study, stoats were recorded as robbing just over half of 149 birds’ nests in bush near Kaikōura. They make off with most bush pigeon and kākā eggs and nestlings. Stoats have short lives and highly variable birth and death rates – their population can rapidly increase when food becomes abundant.
Captain James Cook and later whalers and sealers introduced cats (Felis catus), but these did not become common until the 1830s. In the 1870s, rabbit numbers were driving farmers from their land, so quantities of cats were released to control them.
Cats were carried on ships to control rats. When James Cook’s Resolution was tied to trees in Dusky Sound, Fiordland, in 1773, one of the cats ‘regularly took a walk in the woods every morning and made great havoc among the little birds, that were not aware of such an insidious enemy.’ 1
The cats went into the bush, joining rats and stoats as predators. Soon after cats appeared on Little Barrier, Cuvier and Stephens islands, saddlebacks and other native birds disappeared. Introduced to forest-covered Herekopare Island, cats quickly extinguished parakeets, robins, fernbirds, brown creepers, snipe and a native bat.
Wild cats live high in the mountains and along the coast, as well as in bush, scrub, and on farmland. They feed mainly on young rabbits, rats and mice, but also on native birds, lizards and large insects such as wētā, cicadas and dragonflies. In mainland bush, birds make up some 15% of their diet but, with most native birds gone, they usually eat blackbirds, chaffinches, silvereyes and hedge sparrows.
It is difficult and expensive to rid the bush of cats. It took 128 people almost 400 days to remove 100 cats from Little Barrier Island. In some cases they can be seen as a lesser evil, as they prey on rats, which cause even more damage.
Other introduced pests such as weasels, ferrets and hedgehogs also live in the bush but make lesser inroads into the native animal populations than cats and stoats.