The rise of the field vole

Gamekeeper Simon Lester reports on the field vole, a tiny terror that's prepared to kill interlopers.

Anyone out and about in the countryside during the past 12 months might be forgiven for thinking their eyes were playing tricks on them due to the little brown flashes here, there and everywhere. There’s no need to consult an optician what you’re seeing is field voles and 2014 was a bumper year for our most common mammal. On closer examination, you will notice that grassland is currently pockmarked with multitudinous tiny runs and holes as an estimated 75 million field voles throughout the country go about their business.

Also known as the short-tailed vole because it has a smaller stern at 1½in long than the bank vole, Microtus agrestis’s body measures just 5in. It shares many of the appealing features of its cousin the water vole: the blunt, chubby face and furry ears that blend into long, fluffy brown-grey hair and make them look less rodent-like.

The field vole frequents many habitats, but does well in ungrazed or lightly grazed grassland. They are almost exclusively herbivorous and prefer to eat grass, but also like nibbling on other vegetation, such as tree bark. When I’m checking my traps on the hill, I often see the bright, white strips of pith, like small piles of spaghetti, that they’ve cleverly stripped from the middle of rushes, having devoured the juicy green outer layer. There is always a latrine nearby, with piles of shiny, green droppings.

Although they burrow in soil, field voles choose to nest above ground in small, round nests of chewed grass, usually at the base of a protective tussock of grass or a log. They love to create a world of their own, with a network of tunnels and holes under the layer of thick thatch, each animal aggressively and noisily, with much squeaking and chattering defending its tiny territory. If you place a board or a piece of corrugated iron on the ground, they will quickly nest underneath it.

As they mate between March and October, with a 21-day gestation period, and can have an astonishing seven litters a year, with up to seven young in each that’s up to 49 babies it’s easy to see why their numbers soar so quickly when they’re at the top of their cycle, which is usually every three to four years.

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There are various theories as to what causes the field-vole population to fluctuate so wildly when food becomes scarce. It was once thought that predation by weasels was one reason, but this was disproved by a recent study at Kielder Forest in Northumberland, which concluded it was more likely to be due to diseases such as cowpox or TB. The truth is that it’s tough being a vole because most other animals want to eat you.

This is why such a miniscule creature can exert such a big influence on Nature, driving massive increases in the populations of a long list of specialist predators, such as kestrels, barn owls and short-eared owls, whose very existence depends on small mammals, voles in particular, to sustain them over the lean winter months.

Hen harriers are said to be able to see voles’ urine trails from their vantage points high in the sky and the local vole population is a big deciding factor in where these raptors choose to settle to breed, as they feast on voles before passerines return to the uplands in spring. More generalist predators, such as foxes, buzzards, stoats and weasels, don’t depend as much on voles, but they do see a massive increase in productivity and survival rates of their young during good vole years.

Foxes love volesI’ve examined the stomach contents of dozens this year and many have been stuffed with voles. Most dogs are also excited by their scentour terrier, Chester, regularly snaffles them out of their cosy, grassy cocoons.

Our resident barn owls benefit from the high numbers of field voles that abound in our garden and the tall grass in the meadows surrounding the house. It only takes the barn owl about 5–10 minutes to locate a vole and take it back to its young no wonder then, given the field vole’s population explosion last year, that barn-owl numbers have also boomed.

The heron is also an adept field-vole fancier. Standing ankle deep in grass in front of our house, instead of the river at the bottom of the valley, he adopts the same stalking procedure and, with arrow-like precision, will quickly have a vole in his stabbing beak. I’ve found several large pellets of solid vole fur that the herons have regurgitated.

Apart from being the currency of the uplands and fuelling so much in Nature, field voles can become a pest when they reach the numbers they did in 2014. Lawns, football pitches and shrubs, but most of all forestry and orchards, all suffer from their talent for tunnelling and their fondness for stripping the bark off small trees, both above and below ground level. For the first time, I’ve seen plastic vole guards fitted to young trees this year, a sure sign that the tiny terrors are making their presence felt.

There are about 75 million field voles scurrying about the UK and the females can produce up to 49 young each year.

Voles on patrol
• A vole will attack, and even kill, another vole trespassing on its territory.
• Although largely nocturnal, in winter, they can be seen foraging during the day to cope with reduced food supplies.
• Their populations usually peak on a four-year cycle.
• Always wary due to the threat of aerial predation, voles will often stop to sniff the air and stand up on their hind legs to look around.
• They have poor eyesight, but highly attuned hearing, quickly darting into cover at the first sign of danger.
• Surprisingly vocal, field voles make lots of loud chirping and chattering noises.