Mustelids in Northland – Ferrets, Stoats and Weasels

Mustelids ( Ferrets, Stoats and Weasels) Arrival in New Zealand,

• Ferret Mustela furo
• Stoat Mustela ermina
• Weasel Mustela nivalis

All three were introduced into New Zealand in the late 1880’s as a means of controlling the rabbit population that was getting out of hand and destroying farmland. While they are expert rabbit hunters and killer, they found the native wildlife to be much easier targets.

In the mid-1980s Ferrets were farmed for their fur and called ‘fitches.’ At least 17 ferret farms were established in Northland. When the fur trade stopped being profitable and the farms closed down many fitches escaped or were set free, helping ferret numbers to expand in the north. Ferrets adapted well to the New Zealand environment and New Zealand currently has the biggest wild ferret population in the world. It is now against the law to keep ferrets in New Zealand.

Mustelids are small, secretive, fast-moving and often nocturnal so are seldom seen, but that does not mean that they are not around and having a major impact on local native species.

Mustelid Terminology

Ferrets

Being farmed animals, there is a whole range of terminology for describing Ferrets, or ‘fitches’;

• A male Ferret is called a ‘hob,
• A female ferret is called a ‘jill’,A spayed female ferret is called a ‘sprite,
• A neutered male ferret is called a ‘gib’,
• A vasectomised male is called a ‘hoblet’,
• Young ferrets, less than a year old, are called ‘kits’.
• A group of ferrets is a ‘business’, or in historic terms, a ‘busyness’.

Stoats

Male stoats are called ‘dogs’, ‘hobs’, or ‘jacks’.

Female stoats are called ‘jills’.

Young stoats are called ‘kits’, the same as young ferrets and weasels.

A group of stoats are called either a ‘gang’ or a ‘pack’.

(note difference in tails between weasels and stoats)

Weasels

Male weasels are called ‘dogs’ and

Female weasel are called ‘bitches’.

Young weasels are called ‘kits’, the same as young ferrets and stoats.

A group of weasels are called either a ‘gang’, a ‘pack’, a ‘boggle’ or, the best description of all, a ‘confusion’.

Description of New Zealand’s Mustelids

Ferrets are the easiest of the three to identify due to their large size.

Stoats and Weasels are often confused. The two key differences to look for are;

• Size: Stoats are bigger than Weasels,
• Tails: Stoats have a black tip at the end of their tails that Weasels do not.

The table below summarizes the differences between the three types;

Mustelid Habitat

Stoats are the most common of New Zealand’s mustelids because they are found in most habitats and have adapted well to living in New Zealand bush. Ferrets and weasels are more often found in open country like farmland rather than in the bush.

Mustelid Diet

Mustelids are nocturnal carnivores. They like to feed on small animals such as rabbits, rodents, possums and birds. They will also take easier prey like lizards, frogs, insects and bird’s eggs.

Weasels

The weasels super-fast metabolism means they have to eat about half their body weight each day. This makes them very aggressive hunters.

Weasels will corner and grab their prey, wrapping their muscular body around the animal to immobilize it and then delivery a single killing bite to the back of the head, puncturing the skull or spinal cord.

The weasels killing instinct is triggered by movement. Even with a full belly they will attack anything that moves and looks like prey, often killing much more than they can possibly eat. To a weasel most things look like prey. They are quite capable of killing and carrying off animals up to four time their size.

Stoats

Stoats are extremely aggressive, energetic and versatile. They will investigate every hole, under any covering and their expert climbing skills will take them to the top of the tallest trees. They are also very good swimmers, swimming distances of up to 1.5km. This would have helped them spread throughout New Zealand and access in-shore islands.

Mustelid Breeding

Ferrets

Female ferrets reach puberty at 8 to 12 months old. Ferrets will have between three and seven kits, two or three times a year, normally between September and March.

Pregnancy lasts around 42 days. The kits are weaned between three and six weeks after birth and become independent at about three months old.

Stoats

Stoats are unusual in that the females become sexually mature at two to three weeks old while still blind, deaf and hairless. They are usually mated with by adult males before even being weaned. Males become sexually mature much later at ten to eleven months old.

Stoats only have one litter a year. After mating, the pregnancy is delayed for 280 days. This is known as ‘delayed implantation’. The gestation period after this is only 21 to 28 days and results in a littler of between 5 and 12 kits. Within four weeks the kits are eating solid food, after five or six weeks their eyes open and at six to seven weeks their black tail-tips appear. After 12 weeks the kits are weaned.

The average life span of a Stoat is only one to one and half years in the wild but they can live for up to seven years in captivity.

Weasels

Each weasel has a territory of between four and eight hectares, depending on the food supply available. Females territories tend to be smaller than males and may overlap with the territories of males. Territories are marked with a strong smelling spray from the anal scent glands, much the same way male cats mark their territories.

Breeding season is between September and March and is the only time the males and females will associate with each other. Females will not leave their territories but will wait for the males to find them.

They can produce either one or two litters per year, each with four to six kits. Gestation takes about five weeks. The kits are weaned at about four weeks old and are able to hunt and kill between five and eight weeks old. They will often learn by accompanying their mother on hunting trips.

The average lifespan of a weasel in the wild is one to two years.

Mustelid’s Effect in the Tangihua Forest

Mustelids have contributed to the extinction of range of New Zealand native birds including Huia, Brush Wrens, Native Thrushes and Laughing Owls. They are also part of the reason that birds such as Saddleback, Stitchbirds , Kākāpo and the Little Spotted Kiwi are not able to survive on the mainland and are now restricted to predator-free off-shore islands.

New Zealand did not have any mammalian predators until humans arrived, so our native birds did not develop any means of defending themselves from these threats. A high proportion of our native birds are flightless and nest on the ground, making themextremely vulnerable to mustelids.

While ferrets and weasels are a major threat to New Zealand natives, it is stoats that have caused the most damage as they have adapted so well to most of New Zealand’s environments.

Birds that nest in holes, such as Yellowhead and Kākāriki, are the most vulnerable. The stoat can easily enter the nest and take the eggs, chicks and the adult bird on the nest as there is no escape route.

Even in low numbers, stoats can have a huge impact on kiwi survival. Stoats kill an average of 10 North Island Brown Kiwi chicks every day, equating to 3,650 chicks each year, or 40% of all North sland Brown Kiwi chicks.

Due to mustelids and other predators, only 12% of North Island Brown Kiwi chicks make it to six months old. At this age they are large enough to defend themselves from mustelids. Stoats are the main reason the Tangihua kiwi population has become functionally extinct.

Conservation Plan for Reducing the Number of Mustelids

Controlling mustelids in the Tangihua Forest and wider New Zealand will make a significant difference to the chance of Kiwi surviving and thriving.

Mustelids are one of the harder pest species to control or eliminate. Unlike Rats and Possums, they need to be trapped rather than poisoned. This means that, instead of putting out poison every few months, traps have to be set and checked every few weeks.

Mustelids are also very clever and will become ‘trap-shy’ if the trap goes off and gives them a fright but doesn’t catch them. To combat this, we use special box traps with extremely sensitive Doc 200 humane kill traps. Refer to our Bait Stations and Traps page for more information on these and how we are using them in the Tangihua Forest.

General Conservation Plan in the Tangihua Forest

• Tangihua Community Pest Control Area: Targeting pests on farmland with local farmers and the Northland Regional Council
• Conservation on DOC Land: Maps of the Tangihua Lions Lodge Conservation and Education project
• Conservation Education: Creating a high quality, accessible educational center surrounded by a fully functional ecosystem for schools and anyone else who may choose to use the facility.
• Volunteers Options: How you can help protect our native bush.