Around 250 mm in length and weighing about 100 g the Weasel Mustela nivalis is the smallest British Mustelid. The body fur is ginger to russet brown with a cream belly. A Weasel’s life span is short, with most of the Weasel population at any one time being under one year in age and only one in 80-90 young surviving over two years (Corbet and Harris, 1991).
The Weasel is widespread throughout Britain but absent from Ireland and many offshore islands, implying that it colonised Britain later than the Stoat Mustela erminea after the last glaciation. It is probably our most numerous carnivore; often seen crossing roads and lanes by drivers. Like its close relation the Stoat, the Weasel is found over a wide range of habitats including urban areas, frequently utilising hedgerows, stone walls and other linear features that have good supplies of small mammals (Harris and Yalden, 2008). The author has recorded them in such varied landscapes as the Pennines (Nenthead), in sand dunes (Druridge Bay and Seaton Carew), coastal marsh (Cowpen Bewley), former colliery spoil heaps (Murton and Herrington) and the embankments of the A19 at Sunderland.
Diet is mostly small mammals up to about the size of a young Rabbit Oryctolagus cuniculus but birds, bird’s eggs, reptiles, amphibians and earthworms are also occasionally eaten (Harris and Yalden, 2008).
The Weasel specialises in hunting small tunnel-living prey such as voles and mice. Its small size means it can hunt them both above and below ground. Although they are mainly diurnal they will sometimes hunt at night. They do not hibernate and actively hunt under the cover of snow. Like its cousin the Stoat it will often take over the dens of its prey and Weasels will have several dens within their range (Harris and Yalden, 2008). The author has had a Weasel take up residence in his garden compost heaps to prey on the abundant mouse population around the nearby chickens. The removal of one compost heap recently revealed a den complete with mummified food reserves. They are good swimmers and the author once watched one catching voles and storing them as flood waters receded by the river Wear near St John’s Chapel.
Weasels follow the typical Mustelid territorial pattern with exclusive male territories encompassing overlapping female territories. Weasel home ranges fluctuate greatly depending on the distribution and density of prey. Resident animals may defend their range when numbers are high and neighbours numerous, but in the spring the system breaks down as males prospect widely for females (Harris and Yalden, 2008).
In the UK Weasels are reported not to turn white in winter like the Stoat although it does so elsewhere in its range. According to Flintoff (1935) it does occasionally turn white in UK winters but this is open to debate. Mr W. Walton records an albino Weasel in upper Teesdale in The Victoria History of the County of Durham (Page, 1905).
Weasels normally produce one litter a year sometimes two if Field Vole Microtus agrestis numbers are high. Typically four to eight youngsters are born and are weaned at three to four weeks; they can kill efficiently at eight weeks and split from the family group between nine and 12 weeks. In a good vole year females can be pregnant at three to four months old (Harris and Yalden, 2008).
Food shortage is the main killer of young Weasels as they must eat up to a third of their bodyweight a day. The author once followed Weasel tracks in the snow in Herrington, Sunderland for about 500 metres noticing how the animal followed the vole tunnels under the snow, surfacing often then disappearing again. Eventually the little Weasel was found curled up dead at the base of a tree: starvation was assumed to be the cause of death. Terry Coult (pers. comm., 2012) recalls seeing a Weasel in the open and behaving in an erratic manner near Langley Park, which on examination turned out to be infected with the nematode parasite Skrajabingylus nasicola, with significant damage to the skull. Weasels are trapped by gamekeepers as part of their predator control programmes but not specifically targeted as they are seen as less of a pest than the Stoat. The author has seen them on many a keeper’s gibbet in Teesdale, Durham, Hexham and Corbridge. The gamekeeper’s trap is the major limit on the population but the species occasionally falls prey to hawks, owls, Fox Vulpes vulpes, Mink Neovison vison and even cats.
There is little competition between Stoats and Weasels: as the Stoat is larger and much stronger it takes larger prey like Rabbit whilst its smaller cousin is an out and out “tunnel hunter” reaching small rodents the Stoat cannot. Weasels rarely venture into the open to hunt, sticking to cover to protect them from aerial predation and foxes (Harris and Yalden, 2008).
The Weasel is circumboreal in distribution, found around almost the entire northern hemisphere including large parts of the Arctic Circle, but excluding the larger islands such as Ellesmere and Greenland. It is sympatric with the Stoat for most of its range but extends further south in the Mediterranean and into North Africa. It occurs throughout mainland Britain but is absent from Ireland (Harris and Yalden, 2008).
Except in the uplands its distribution in the northeast of England is ubiquitous and it can be found from the Tweed to the Tees; it appears to be more abundant in lowland areas and nearer the coast than the Stoat. Recent records from the Wildwatch project in the North Pennines Area of Outstanding Natural Beauty (AONB) show 25 Stoat records to three Weasel records within the AONB boundary. The author did however come across a family party of Weasels by the roadside in the summer of 2004 near Nenthead, showing they are not completely absent from the uplands. Our distribution maps post-2000 probably reflect the recording effort as opposed to actual distribution, as the species is much more widespread than is indicated.
An aborigine, its remains are common in cave deposits across Europe (Harris and Yalden, 2008) including the Teesdale Cave (Simms, 1974). Historically the species has always been abundant; Mennell and Perkins (1864) make only a one-line statement “this animal is very abundant throughout our district.” Hutchinson (1840) records the unlikely incident of a pack of seven Weasels hunting a Brown Hare Lepus capensis by scent on Upper Houses Farm, Lanchester. Even more unlikely, in 1824 William Henderson describes how he and two other boys were hunted by a pack of at least 20 Weasels near Windlestone in Durham, only escaping by seeking refuge in the nearby village (Apperley, 1926).
Weasels are often caught whilst surveying for small mammals and Don Griss (pers. comm., 2012) reports an irate Weasel caught when trapping in the ICI reed beds near Billingham in the 1990s.
The estimated British pre-breeding population is put at around 450,000, although this could vary considerably; actual populations are extremely difficult to predict as they are related to food supply (Harris and Yalden, 2008). The Weasel is still considered to be common but census difficulties and natural fluctuation in numbers make it hard to predict whether there is a conservation concern for the species.
Written by Kevin O’Hara (Last updated Nov 12)