Stoats: New Zealand’s Stealthy Threat to Native Birds

During the late 19th century, stoats were brought to New Zealand alongside ferrets and weasels in an effort to reduce the country's rabbit population. New Zealand scientists, including the well-known ornithologist Walter Buller, warned against introducing these predators, but their concerns were not heeded. Once released, stoats quickly began causing serious damage to native bird populations.

Their ability to control rabbits was considered valuable enough that stoats, ferrets, and weasels were legally protected until 1936. Despite the growing evidence of their impact on native wildlife, these introduced predators continued to receive protection for many years.

What Do Stoats Look Like?

Stoats are members of the mustelid family, the same group that includes weasels and ferrets. In terms of body size, they fall between the two, being larger than weasels but smaller than ferrets.

They have a distinctive bushy tail with a black tip. Their bodies are generally covered in dark brown or blackish fur, while the underside of the body and throat are much paler.

Where Do Stoats Live?

Stoats are now found throughout Aotearoa New Zealand. They are highly adaptable and can establish themselves almost anywhere that provides enough prey.

Their habitats include high-country landscapes, coastal and beach areas, farmland, native bush, and forests. They are also capable of surviving at high elevations, including areas above the tree line.

What Is Stoat Behavior Like?

Stoats possess the excellent eyesight, strong hearing, and highly developed sense of smell typical of mustelids. They can remain active both during the day and at night and are fast-moving predators.

Their climbing ability allows them to reach nests in trees and prey on young birds and eggs. They are also thorough hunters, inspecting burrows, holes, and other hiding places they encounter. When they discover a ground-nesting bird, the bird often has little opportunity to escape.

Stoats do not always stop hunting after obtaining enough food for themselves. When circumstances allow, they may kill more prey than they immediately need and store the excess inside their dens for later consumption. They are capable of killing animals considerably larger than themselves and generally keep their den locations well hidden.

These predators can also cover impressive distances. One young stoat was recorded traveling 70 kilometers in only two weeks. They are capable swimmers as well and can cross at least 1 kilometer of water to reach offshore islands.

Stoats are intelligent and cautious animals, which makes controlling them particularly difficult. They can be wary of unfamiliar bait and traps, allowing them to avoid many attempts at capture.

What Impact Do Stoats Have on Native Wildlife?

New Zealand's Department of Conservation, or DOC, has described stoats as “public enemy number one” for the country's native birds. These persistent predators place considerable pressure on many native species, including wrybills, New Zealand dotterels, black-fronted terns, and young kiwi.

The survival rate of brown kiwi chicks in Northland is especially low. Only around 5 percent survive to adulthood, with DOC identifying predation by stoats and feral cats as their greatest threats.

Stoats also pose a danger to birds that nest inside tree cavities. Species such as mohua, kākā, and yellow-crowned kākāriki can be particularly vulnerable because their nesting sites provide access points for these skilled predators.