A remarkable meal

Spend enough time watching wildlife and you will be guaranteed occasions that surprise, impress and delight you. Just the other day I chanced across one such occasion. Scanning across the marshes I picked out a heron struggling with an unusually large prey item. The bird was distant and the magnification of the binoculars insufficient to resolve what it was that the heron was struggling to swallow. Quickly setting up my telescope I was soon on the bird. Much to my amazement the heron was attempting to swallow a stoat, the rear two-thirds of which were hanging from the bird’s beak.

The stoat was dead, of that there was no question; the russet body and black-tipped tail hung limply, swinging pendulum-like as the heron attempted to reposition its prey. Over the following minutes the heron continued in its efforts to swallow the stoat, throwing its head back and pointing its bill skywards, as if seeking the aid of gravity to push the unfortunate mustelid further down its throat. Finally, after much effort, the last of the stoat slipped out of sight and the great bill closed.

This, however, was not the end of the struggle. The bulge of the stoat could be clearly seen in the throat and the heron again began to throw back its head. Such a large prey item was causing discomfort and the bird still had some way to go to get its meal further down the digestive tract. Moments of continued effort were followed by periods where the bird stood still, only the great bill opening and closing as if the heron was struggling to draw breath. Then, the act of swallowing still unfinished, the bird walked down the bank, into a drain and out of view.

Whether the heron had stumbled across the stoat, already dead, or whether the stoat had wandered unknowingly within range of the heron’s dagger-like beak, was unclear. I suspect the latter but whatever had happened prior to my arrival it was still a remarkable sight. Herons will take larger fish and they regularly take small birds and mammals but this was something that I’ve not seen mentioned in the literature. It was new to me and all the more astonishing for it.

Stoat

Normally alert to human approach, the Stoat must have been unaware of me as I skirted the edge of the Rabbit warren that is a bustle of activity at this time of the year. It was not until I was within just a few feet that the Stoat dived for cover, leaving me to settle and wait in case its inquisitive nature afforded me with a second, more prolonged viewing. At this time of the year Rabbits dominate Stoat diet, with male Stoats in particular targeting them, while females tend to take more small mammals, like mice and voles.

Prey preferences reflect the size difference that exists between male and female Stoats; males are the larger of the two sexes and, at about 30cm in length, this one was undoubtedly a male. This makes males better equipped to deal with larger prey, like Rabbits, although both sexes will tackle prey species that are decidedly larger than themselves.

A dominant male Stoat will hold a territory that overlaps with several (smaller) female territories and during the breeding season he may expand the size of this by up to fifty-fold. During this period, which peaks between April and June, he will seek out the females, spending a few days with each, before moving on to find another. Female Stoats use delayed implantation of the fertilized egg, meaning that young are not born until the following Spring.

Sometimes when I am out and about I get the opportunity to watch a Stoat hunting, as it moves through an area and searches burrows and other forms of cover within which potential prey may be found. From time to time, a Stoat will stand on its hind legs, a behaviour known as periscoping, presumably to get a better view of the surroundings. I have also witnessed encounters where a Stoat has taken a Rabbit, sometimes with a quick charge but other times seemingly able to mesmerise its prey. Some individuals may be seen to exhibit a wild, cavorting dance, but whether this is a behaviour used to mesmerise prey or the result of an infection of a nematode worm is unclear. This particularly unpleasant parasite is thought to occur in up to a third of British Stoats, infesting the sinuses and bringing about skull deformity and, presumably, other internal damage.

This Stoat was showing no signs of emerging from the cover into which it had dived and I attempted to imitate the squeaking and squealing of an injured Rabbit. This sometimes brings them out into the open, a trick that an old gamekeeper friend once taught me. Either the Stoat was not playing ball or, more likely, my squealing Rabbit impression was not up to scratch.

Where are the Stoats in Ermine

By all accounts it has been a cold winter, with temperatures across the county dipping to their lowest level for many years. Under such freezing conditions you might expect there to be a few records of Stoats in ermine, the white winter fur that has long been valued as regalia for nobility and the judiciary. However, here in Norfolk the record of a Stoat in ermine continues to be a very rare occurrence. As we shall see, there is a good reason for this.

The white winter coat of a Stoat is a feature of northerly populations, the brown fur replaced with white during the autumn moult and only the black tip to the tail retaining its colour. In temperate latitudes, such as our own with its variable climate, the moult into ermine is most frequently seen in Scotland, Wales and Northern England. For example, in the extreme northeast of Scotland, some 90% of adult Stoats turn white in winter. Further south, there are occasional reports of individuals in ermine, together with others which may appear mottled.

Temperature at the time of moult has an influence on whether or not an individual Stoat will go into ermine. Research has demonstrated that there is some form of chemical switch in the Stoat’s brain, triggered by temperature, which determines whether or not the new coat is white or brown. The switch determines whether or not melanin is produced in the growing fur and, in its absence, the fur is devoid of colour (i.e. it is white). Since fur on different parts of the body is replaced at different stages of the autumn moult, and because the switch changes position (on or off) with changes in temperature, it is possible for some tracts of the coat to emerge brown and others white. It is this that gives rise to the mottled individuals that are sometimes seen. Here, in Norfolk, the temperatures at the time of moult are normally sufficiently warm enough to just produce brown coloured winter coats, and so Stoats in ermine remain rare.

However, it is not quite as simple as this, because there is also a genetic component to the transition into ermine. Clever experiments, one translocating a Stoat from the Alps to England, and another keeping a Stoat from a northerly latitude in a warm room, illustrated that temperature does not act alone. More recent studies have confirmed this and have demonstrated that there is a hereditary basis to coat colour that is also sex-linked. This results in females being more frequently found in ermine than males. Having a white coat is advantageous in cold, snow-covered latitudes, providing both protection from predators and better insulation. Further south, a brown coat is advantageous.

Watch out there's a stoat about

The Stoat is one of those animals that I only see occasionally, perhaps running across a track or working the base of a hedgerow in search of small mammals or other prey. As such, I tend to take notice of where and when I see Stoats, noting them in my field book to report at the end of the year. It seems that I am not alone in this; look at the Norfolk Mammal Report (available from the Norfolk & Norwich Naturalists’ Society) and you will see the species features more often than others that you would consider to be more common across the county. Does familiarity breed contempt and are we ignoring the commonplace?

There is something mesmerising about a Stoat that is working an area in search of prey. Ever alert, it searches opportunistically, pausing every now and then to scan for other predators and would-be prey. I have only seen a Stoat tackle prey on a handful of occasions, usually a vole or mouse, but on once a Rabbit considerably larger than the Stoat in size. Stoats are reputed to mesmerise their prey by dancing but this is unlikely to be the case. The few incidents where observers have reported such behaviour may, in fact, be observations of a Stoat suffering from an infestation of a parasitic nematode, which causes skull deformity and spasms.

Although the Stoat is one of the smaller European carnivores, it is not the smallest and shares the British part of its wide range with the even smaller Weasel. The two species can be told apart fairly readily; the Stoat larger, with a long black-tipped tail; the smaller Weasel with a shorter tail and no black tip. Additionally, in Stoat, the chestnut back and flanks meet the cream belly in a straight line, whereas in Weasel this line is irregular and sometimes spotted.

There is one other aspect of Stoat ecology that fascinates me and that is its breeding system. Both male and female Stoats are territorial, with each male holding a territory within which there may be one or more smaller female territories. During the breeding season each male will adopt one of three strategies, the choice dependent upon his age and social status. Older dominant males adopt a roaming strategy, expanding the size of their territory massively. They then roam around the territory in search of females, spending a few days with each before moving on. Younger males stay within their smaller territories and the youngest males remain fully transient, unable to hold a territory of their own. Although mating occurs from April to June, the females delay implantation, so the resulting young are not born until the following spring.

Hair trigger mouse traps with teeth

Stoats and weasels are two of our most widely distributed mammal species, familiar to most but only rarely seen well in the field. Their small size, use of available cover and energetic manner mean that the typical view is of one dashing across a quiet country lane with an arched-backed gallop, in the case of the stoat, or as if on tiny wheels, in the case of the weasel. These two carnivores are beautifully equipped for the pursuit and capture of their favoured small mammal prey. The small, almost cylindrical bodies and short legs enable them to chase small mammals into their burrow systems. The degree of efficiency associated with their ability to hunt small mammals even led one researcher to refer to them as “hair-trigger mouse-traps with teeth”, a suitably apt description.

Both species have long fascinated me. First introduced to them through a gamekeeper friend, I enjoyed many encounters with both species when I spent three years live-trapping small mammals on a north Norfolk estate. Part of my study site was well used by at least one stoat and he would sometimes stand on his back legs and stare at me intently from a short distance away. Even closer encounters were achieved with the local weasels, some of which got into the habit of entering my small mammal traps. My normal procedure for emptying these involved tipping the contents into a clear plastic bag so that the mouse, vole or shrew could be aged, sexed, weighed and marked before being released. Dropping an unexpected and angry weasel into such a bag is not something I would recommend! One such individual stood at my feet and berated me for several minutes upon release and, despite it’s tiny size, was actually rather intimidating.

Stoats may be equally intimidating and are commonly believed to mesmerise rabbits, reducing them to dithering and helpless wrecks, before delivering the killing bite. Examination of rabbits killed by stoats does reinforce the view that some of these victims have literally died from fright. Another curious behaviour associated with both stoats and weasels is the manic somersaulting and leaping about that is sometimes witnessed by observers. Some have suggested that this behaviour is either a form of play or is a trick to catch overly curious prey. However, it may actually be linked to a parasitic nematode worm that goes by the tongue-twisting name of Skrjabingylus nasicola. This bright red worm occupies the sinus cavities of a fair proportion of both species, causing damage to the skull and, potentially, the brain. It could be this that, through irritation, causes the “dancing”. Curiously, the worm spends part of its life cycle in small mammals, further strengthening the link between predator and prey.