Research
Non-invasive methods for monitoring weasels: emerging technologies and priorities for future research (2024) DS Jachowski et al
Species Weasel
Facts about weasels
- English name: Weasel
- Scientific Name: Mustela nivalis
- Number of young: Two litters of three to six kits born per year
- Diet: Small mammals such as voles and mice, as well as small birds
- Habitat: Variety of habitats: woodland, grassland, hedgerows, heathland and moorland.
Distribution and status
The weasel (Mustela nivalis) is widespread and thought to be relatively common across Britain and is classified as 'Least Concern' in England, Scotland and Wales. Weasels are absent from Ireland as well as from most British islands.
There is a need to develop an effective survey methodology to monitor population trends in the species.
The weasel is the smallest of the small mustelids and the smallest of all the British carnivores.
Weasels have short legs and a slender body (17-24cm). Their fur is chestnut brown on the back and head with a creamy white belly, and the division between brown and cream is irregular and can be spotted in appearance. This irregular pattern is different for each animal so can be used to identify individuals. The tail is short (3-6cm) and is a uniform chestnut brown colour without a black tip. Unlike stoats, weasels don't change fur colour in winter.
Stoats and weasels look very similar. The main differences are:
- Stoats have a black-tipped tail
- Weasels have an irregular back-belly line, whereas stoats have a straight back-belly line
Weasels are found in a wide range of habitat that offer prey availability and good cover for hunting, including lowland pasture, woodland, marshes and moorland. They are less common where their prey is scarce, such as at higher altitudes and in woodlands with sparse ground cover.
Weasels usually take over the nest sites of their former prey and often have several den sites within their territory. Territories can vary in size depending on habitat quality and food availability. Males and females live in separate territories, with males territories overlapping female territories.
Weasels largely hunt small rodents, such as mice and voles, but will also take birds, eggs and young rabbits, particularly if rodents are scarce.
They almost exclusively eat rodents in summer and autumn and in spring, juvenile rabbits are an important food source. Shrews, rats, reptiles, amphibians, invertebrates including earthworms make up a small proportion of their diet.
The weasel’s small size enables it to search through tunnels and runways of mice and voles and means they can hunt at any time of the day or year. Despite their size, weasels can hunt prey much larger than themselves, such as rabbits and rats.
They do not hibernate and can hunt even under deep snow. The main disadvantage in being so small is that weasels must eat a third of their body weight every day just to stay alive.
Mating occurs April – July and implantation is direct, unlike some other mustelids that have delayed-implantation and therefore delayed pregnancy. Pregnancy lasts an average of 36 days and between 4-6 kits are born May–August.
Kits are born pink, blind and deaf and weighing just 1.5-4.5 grams. They are dependent on their mother, who rears them alone. Kits are weaned at 3-4 weeks and can kill efficiently at 8 weeks. Family groups split up at 9–12 weeks. Weasels can breed in their first year.
When voles are abundant, weasels can have two litters per year, but they usually have one litter. In poor vole years, they may not breed at all. Due to their quick reproduction, they can recover their numbers quickly after periods of low food availability, and as a result are one of the most successful carnivores.
Weasels have no legal protection in Britain, and their elusive nature makes them difficult to monitor.
The main predators of weasels are foxes, owls, kestrels and cats but have also been considered enemies of gamebirds and controlled by gamekeepers.
Other significant threats include persecution (control by gamekeepers), habitat loss and road casualties.
Traditionally, gamebag data collected by gamekeepers have been used to record the distribution and abundance of weasels, although the accuracy of these data has been questioned. There is a need to develop a more effective survey methodology to monitor population trends in the species.
Populations of weasels can experience local extinctions, but they are good at recolonising when conditions improve.
Our work
As part of the MISE Project, Vincent Wildlife Trust developed baited hair-tubes, which were set up in a variety of locations and habitats to detect weasels in west Wales .
We have more recently been trialing methods to study weasels and have worked in collaboration with the Dutch Small Mustelid Foundation to trial the ‘Mostela’, which was developed in the Netherlands to monitor weasels and stoats. Watch the video of some of the mustelid footage we’ve recorded in the Mostelas.
Further reading
Research
Croose, E. & Carter, S.P. (2019). A pilot study of a novel method to monitor weasels (Mustela nivalis) and stoats (M. erminea) in Britain
Information leaflet
Guide to Identifying the Small Mustelids of Britain and Ireland
Information chart
A quick-start guide to the identification of small mustelids
FAQs about weasels
Weasels are our smallest carnivore and are specialised small mammal hunters. Like our other carnivores, they are opportunistic and may take small poultry, but this is likely to be an insignificant part of their diet due to the energy costs involved in hunting prey items much larger than themselves.
Weasels in groups are most likely to be a family group – a mother and kits, but they split up at 9-12 weeks. Groups of weasels do not work together to take larger prey items.
The weasel’s natural predators include domestic cats, foxes, owls, and birds of prey.
No, weasels never turn white... so if you see a white weasel, it is likely to be the similar looking but larger stoat, which in some areas, can turn white in winter.