The Baddies - Threats to NZ Native Birds
Mustelids (Stoat, Weasel, Ferret)
By the mid 1870s, rabbits were becoming a serious agricultural pest in New Zealand. Farmers demanded that the natural enemies of the rabbit (mustelids) be introduced. Despite the protests of bird experts, ferrets, stoats and weasels were released throughout pastoral areas and by the mid 1890s they had spread into forests west of Lake Manapouri. Far too late, after many official protests, the government changed its policy on mustelids in 1903. However, it was not until 1936 that all legal protection for mustelids was removed.
Stoats are now considered "public enemy number one" for New Zealand birds.
Stoats and weasels are similar in colour and general appearance, but stoats are larger, have longer tails and a straight line where the brown fur on their backs meets the white belly fur. From 350-400mm long from nose to tip of tail, the stoat is reddish-brown above, white to yellowish underneath, and has a long tail relative to that of a weasel, with a distinctive and obvious bushy black tip.Males are generally considerably larger than females. Average weight of males is around 325grams, and females 205grams, though this may vary from region to region.
In very cold, snow-prone areas some stoats develop a pure white winter fur, though this is rare in New Zealand. Stoats have acute sight, hearing and smell, and may have some colour perception.
Agile and excellent climbers, stoats hunt at any time, day or night. Home ranges are generally large 60-200ha, and considerable distances can be travelled in short periods.
A single litter is produced annually of up to 12 kits, from late September to late October. Den sites are well hidden and changed frequently. Young are independent from early January. Nearly all females leaving their natal nest are already pregnant but delay development of the fertilised eggs until the following spring.
They have excellent powers of dispersal and individual juveniles have been known to travel over 70km in two weeks. They are also strong swimmers, known to have crossed water gaps of up to 1.1km to reach islands.
Stoats live in any habitat in which they can find prey. In New Zealand they can be found from beaches to remote high country, at any altitude, up to and beyond the tree-line; in any kind of forest - exotic or native; in scrub, dunes, tussock, and farm pastures. They are even known to occur near human settlements. In open country they are less common than ferrets, but in the forest they are much more common.
The main preys of stoats are rodents, birds, rabbits, hares, possums and insects (particularly weta). Lizards, freshwater crayfish, carrion, birds, eggs, hedgehogs and fish are also taken.
Most stoats (>80%) live less than one year, but adult mortality is lower, and a few may reach 6-8 years of age.
Populations can vary significantly in response to food availability. Large increases in population occur in the year following beech tree 'mast' seeding years due to increased abundance of rodents and possibly also because of bird and invertebrate population growth. Similar increases can occur in podocarp and broadleaf forests during cyclical heavy fruiting seasons.
The ferret is the largest mustelid in New Zealand, with a body length of 320mm-460mm and a tail of 110-180mm. Males are noticeably larger than females, averaging 1.1-1.3kg with females ranging from 400-1100grams. Ferrets are considerably larger and of ‘stockier’ build than stoats.
The colour is variable, with a typical white or cream undercoat and a variable quantity of longer dark guard hairs, giving some animals a black looking appearance while others appear almost white. The tail is uniformly dark. A variable dark mask occurs across the eyes and above the nose.
The principal food of ferrets is rabbits and hares. Densities of ferrets seem to be strongly correlated with density of rabbits. A sudden reduction in rabbit numbers means that hungry ferrets attack other animals like threatened native species. Rodents, possums and ground-dwelling or nesting birds are all frequently taken. Lizards, eels, frogs, insects, eggs, hedgehogs and carrion are minor prey items.
They are mainly nocturnal, with a home range that is variable according to food supply, from 3-70ha. A ferret will usually exclude others of the same sex from its central home range. Scent glands are used extensively to leave territorial scent markers. Mating of ferrets usually occurs in September. The litter, usually of 4-8 (up to 12), is born in October or November, with young independent by late January. Females can have a second litter after this if food is abundant. There is high mortality in the first year, and an average lifespan in the wild may be 4-5 years.
Ferrets are generally absent or in low numbers in areas of high rainfall, where there are few rabbits, or deep within forested areas so there are not too many in Fiordland but they are still present around the township and farming areas.