One of our most voracious predators, stoats are a member of the mustelid family, which includes everything from weasels and ferrets (also in New Zealand) through mink and polecats to otters and wolverines. Most of these species have a dense and luxuriant coat, especially in winter, and have been trapped for centuries for fur.

The stoat’s native range is from Canada to Siberia, and south through all of Europe and Asia including parts of Indonesia. Like most of its relatives, it’s a long, slim animal with a very alert expression and stance. It has a cinnamon-coloured back, creamy throat and belly, and a distinctive black-tipped tail, which differentiates it from the weasel. (In the near Arctic, this coat turns white and is known as ermine.) It’s generally larger than a weasel and has a unique, bounding gait, arching its back as it moves, while weasels run close to the ground.

As predators go, stoats are small, weighing 150–400g and 28–40 cm long, including the tail which is usually about a third of their length.

Although scientists in New Zealand and Britain strenuously opposed their introduction, farmers and the government of the day ignored the science (what’s new?) and encouraged the proliferation of stoats and weasels during the late 1880s, hoping they’d control the previously-introduced rabbits and hares. Within six years, there was a noticeable decline in bird populations, although the rabbits and hares continued to flourish. Astonishingly, they were still protected until 1936.

Stoats are a serious threat to ground-based birds that have evolved without predators. They hunt very efficiently, zig-zagging over their territory, using their first-rate senses of scent, sight and hearing, and are excellent tree climbers, occasionally nesting in trees. Stoats also prey on rats and mice.

One might assume that a season of abundant mast would make life safer for birds. But it increases the risk: all that food makes for a population boom in rodents, meaning ample prey and a surge in stoat numbers. In 2006–07, South Island wild takahē dropped by a third after a plague of stoats, due to an excellent 2005–06 mast season. Where there were no traps, more than half the takahē were wiped out.

Takahē are around 63cm long and 2.7kg in weight, giving an idea of the ferocity of stoats, which can kill rabbits and hares ten times their size. They generally kill prey with a bite to the back of the neck, although larger animals such as rabbits may die of shock, as the stoat’s teeth aren’t sufficiently long to sever the spine. Due to last summer’s rain, we have an abundant mast year this year, meaning an increase in both rodents and mustelids preying on our wildlife.

The sexes live apart and males become sexually mature at 10–11 months.
Females are sexually mature at 2–3 weeks, while still blind, deaf and hairless. Adult males enter the nest to mate with the mother, after which they will mate with female kits. Females delay implantation of the fertilised egg until they have a territory that provides the resources to feed their kits the following spring: thus, almost every female is either pregnant or in heat. They can give birth to as many as twelve young.

If she can’t find good territory, she will reabsorb embryos, or even the entire litter. Males have larger territories, covering several female territories; females tend to stay in the area they were born. In good years, territory is smaller; when resources are scarce males roam farther afield. They live for five years, sometimes longer, and dominant older males can have territories 50 times larger than those of younger, inferior males. Females will mate with multiple partners.

Rodents are the mainstay of stoats’ diets, feeding mainly on birds, including kiwi, kaka, mohua (yellowheads), parakeets, and New Zealand dotterel. In some areas the whio population is now 70% male, due to stoats attacking female ducks while they’re sitting on their nests. Young kiwi are often moved to safe crèches until they weigh a kilogram, by which time they’re large enough to fight off stoats. Where there’s no protection from predators, only 5% of kiwi survive to adulthood; 70% are predated by stoats and cats.

Stoats are excellent swimmers and in a good mast year will swim to islands. They have even managed to cross the 5km to Kapiti Island, where three were killed in 2011. In an (appallingly cruel) experiment in a flume tank, a stoat paddled against a moderate current for two hours without stopping, the equivalent of swimming 1.8km.

I can’t help admiring the stoat, and if I lived in the northern hemisphere, I’d be excited to see one. But on our motu they’re destructive predators and a horrifying threat to our native birds, as well as eating wētā, lizards, freshwater crayfish and fish, all of which are already struggling.

Predator Free Russell has caught 22 stoats and 37 weasels in just over a year.
While this may not sound many, mustelids are very hard to catch. Cameras reveal that only one in five stoat interactions with a trap ends in a catch; they’re incredibly wily. (See picture above.)

It would be wonderful if, by 2050, we could declare them extirpated and watch our wildlife start to recover outside sanctuaries.