The Predators

They’re small, they’re furry, they’re introduced and they’re killing Bluff’s native bird and plant life. Possums, rats, ferrets, stoats, weasels and feral cats all take their toll under cover of darkness. Prior to importation of these animals, the only land mammals native to New Zealand were bats. The lack of land predators is why many native birds were flightless. Introduced predators decimated populations of flightless and those that can fly to the point some species became extinct. BHMET recognises that these mammals are only doing what comes naturally and deserve to be humanely treated; after all, they didn’t ask to be brought to New Zealand. Our Trust keeps an eye on technology so that we ensure our traps and baits are the most humane available.

Possum munching on tree. (photo: Rod Morris)Brush-tailed possums

Possums (Trichosurus vulpecula) damage our native forests by browsing on trees, seedlings and regenerating plants. Their feeding habits can result in some plants, like rata, being eaten out of the forest, or in severe cases it can cause the complete dieback of forest canopy. Possums are also known to predate on native birds, eggs, chicks and native land snails.

Norwegian rats

The Norwegian rat (Rattus norvegicus), also known as the Brown rat, is the largest rat in New Zealand. Norwegian rats are often associated with wet areas such as rivers, streams, drains, lakes, lagoons, swamps and estuaries. They will also occupy buildings where there are suitable nesting conditions and food. The Norwegian rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders. These rats dig well, and often excavate extensive burrow systems. Norwegian rats often dig burrows 60-90 mm in diameter. They are omnivores and will eat almost anything. They’ll catch fish, dive for mollusks and stalk birds.

Ship rats

L-R: Norway, Ship, Kiore and a mouse. (photo: Peter E. Smith from Library of New Zealand)Ship rats (Rattus rattus), also known as the Black rat, are one of the most common mammals in New Zealand, but being shy and nocturnal they are seldom seen. Ship rats do not dig burrows like the Norway rat; instead they use existing refuges. Ship rats are excellent climbers and are better able to exploit bush habitats than Norway rats. Rats are very productive breeders enabling them to rapidly reach large populations when food is abundant. In New Zealand, ship rats have an unusual distribution and importance, in that they are utterly pervasive through native forests, scrublands, and urban parklands. Ship rats reduce forest health through eating flowers, fruit, seed, seedlings, native insects, lizards, bats, birds and their eggs, causing population decline or even extinction. Ship rats are the most frequent predator of small forest birds, invertebrates, and perhaps lizards in New Zealand forests, and are key ecosystem changers.

Kiore rats

The Kiore (Rattus exulans), also known as the Polynesian rat or Pacific rat, is the third most widespread species of rat in the world behind the Norwegian rat and Ship rat. The Kiore has the ability to easily adapt to many different types of environments, from grasslands to forests. These rats are nocturnal like most rodents, and are adept climbers, often nesting in trees. In winter, when food is scarce, they commonly strip bark for consumption and satisfy themselves with plant stems. The Kiore is an omnivorous species, eating seeds, fruit, leaves, bark, insects, earthworms, spiders, lizards, and avian eggs and hatchlings.

Feral cats

Feral cats (Felis sylvestris) are known to predate on New Zealand’s native land and sea birds, as well as various lizard and insect species. Feral cats have generally been found to eat a very broad range of vertebrate and invertebrate prey. Preferred prey usually are small mammals, birds and lizards. It is estimated that feral cats in New Zealand have been responsible for the extinction of six endemic bird species and over 70 localised subspecies as well as depleting bird and lizard species. On Bluff Hill cats pose a significant threat to ground nesting birds like titi and penguins.

Hedgehogs

Hedgehogs (Erinaceus europaeus occidentalis) were introduced to NZ in the 1870s along with many of the other mammalian predators. They are cute and harmless looking, but unfortunately cute and harmless needs to eat. Hedgehogs are mainly insectivorous but are happy to munch on anything including mice, lizards, frogs and bird eggs and chicks. They damage our fragile native ecosystem in two ways: by direct predation on the birds and by gobbling up our native insects, many of which are an essential food sources for the birds and pollinators of the plants. It is estimated that an adult hedgehog consumes an average of 160g of food per day. While the Trust does not actively control hedgehogs, they are sometimes killed in our stoat traps.

Weasel, Stoat and a Ferret. (photo from Northland Regional Council website)Ferrets, stoats and weasels

Ferrets, stoats and weasels (collectively called mustelids) are all bad news for birds. Mustelids are devastating predators that threaten the long term survival of many New Zealand native species. Mustelids will kill almost anything from native bats to freshwater fish. They are major predators of virtually all native birds on Bluff Hill including kaka, kereru, tui, morepork, titi, kakariki, fernbird and shags.

But how do you tell them apart?

Ferrets (Mustela furo) are the largest of the three species in New Zealand, with males weighing around 1kg. Their coat is variable in colour with a mask of dark coloured fur usually present across the eyes.

Stoats (Mustela erminea) are smaller than ferrets with males up to 300 mm long and weigh around 350g. Their fur is reddish brown with a creamy white belly. They have a distinctive bushy black-tipped tail.

Weasels (Mustela nivalis vulgaris) are the smallest and least common mustelid in New Zealand at about 200mm long. Their fur is brown with a white belly often broken by brown spots. Their tail is short, brown and lacks the black tip that the stoat has.