With a body weight of less than 125 g the weasel is our smallest carnivore, but what this diminutive predator lacks in size it more than makes up for in personality. Weasels are perhaps most commonly encountered when seen scurrying across a road or path, displaying the appearance of an elongated vole. The small size is even more apparent when you encounter a female; such is the degree of sexual size dimorphism that the smallest female may weigh barely a third of the largest male, the former measuring in at just 18 cm in length (4 cm of which is tail).
Small body size enables the weasel to target mice and voles within their burrow systems, a place where they are usually free from the risk of predation. Two-thirds of the weasel diet is made up of small mammals, making this a small mammal specialist, with a further quarter comprised of rabbits and the remainder a mix of other mammals, birds and their eggs and, occasionally, earthworms. Rabbits may be particularly important in spring, when young rabbits are widely available and small mammal populations tend to be at their nadir. Being smaller, the females tend to concentrate on small mammals, the male’s larger size allowing a wider range of prey to be taken.
The weasel has a reputation for being ferocious beyond its size and this is certainly not a species you would want to handle. I have caught weasels from time to time in live traps set for small mammals. While they usually rush off once released, I once had one face me down in a stand-off that last for several minutes; the weasel was decidedly nonplussed about having been shut in a trap for several hours. Such ferocity might explain why weasels may sometimes attack prey much larger than themselves. Rabbit has already been mentioned but to this species can be added common rat and even brown hare!
Weasels are easily overlooked beyond such chance encounters. In part this reflects their behaviour, with most of their activity centred on hunting under matted grass or within small mammal burrow systems. They seem less inquisitive than the larger stoat, to which they are related, but will investigate other feeding opportunities when favoured prey are less abundant. For instance, they are known to raid nest boxes erected for birds. At some sites, in certain years, weasels have been a significant cause of nestling mortality.
While small size may offer certain advantages, it does leave the weasel at risk of ending up on the menu for other, larger, predators, such as red fox and domestic cat. I’ve even found the odd weasel jawbone in a barn owl pellet, although it remains rather uncommon.