The Field Vole Microtus agrestis has been present in the British Isles since the last glaciations but is not present in Ireland or some of the western islands. It is a small plump vole (head and body 90-120 mm) with a short tail about a third of the body length. The ears are partly hidden in the fur which can at times appear rough, long and straggly. The colour is grey brown (never chestnut), shading to whitish grey on the belly. Veronica Carnell (pers. comm., 2012) reports that on Lindisfarne, where she never caught Bank Voles Myodes glareolus, some of the Field Voles were of a slightly redder colour than normal. It weighs from 14 to 50 grams.
The Field Vole’s main habitat is rough grassland, but includes young forestry plantations until the ground cover becomes too thin due to it being shaded out by the tree growth. Sparse populations also inhabit woodlands, hedgerows, dunes, moorland etc, but it does not do well on arable land. It has been found on grasslands in the Cairngorms up to 1,300 metres. Their numbers on grassland are related to the grazing level; where grazing is so heavy that no litter layer can develop, numbers will be low due to the lack of material in which to construct surface runs.
They can persist in surprisingly small areas of habitat. When small areas of North Cemetery in the centre of Hartlepool were left uncut it was found that Field Voles were present. As there is no suitable habitat in the surrounding area they must have always been present, perhaps living among tufts of longer grass in an otherwise well-manicured cemetery (I. Bond, pers. comm., 2012). Similarly, works to clear developing vegetation from a small, artificial, shingle island, created for terns in a lake at RSPB Saltholme, found Field Voles present and breeding. To access the island they must have swum a distance of 30 metres then scaled half a metre of vertical revetment board. That this was not an isolated incident was demonstrated when the island was cleared again in a subsequent year (Chris Brown, pers. comm., 2010).
Field Voles are nocturnal, with their main activity at dawn and dusk. Home ranges are based on the nest, which is normally placed at the base of a tussock of grass and is almost indistinguishable from it. It is the centre of a maze of surface runs and burrows in which food stores may be placed. Breeding begins in February and goes on until September. There are several litters per season and up to nine young per litter. The young are ready to mate themselves at the age of six weeks. At the end of the breeding season maturation will be delayed to the following spring. Few, if any, animals over-winter more than once and most of the winter population are of immature animals.
Its main food source is the stems, leaves and roots of grasses but it will nibble at other vegetable matter, for example tree bark, and may eat large numbers of insects. To suit the nature of its diet the teeth are not rooted and grow continuously, which may explain some of the damage it can cause to trees as it has to wear its teeth down to prevent over-growth. Occasional individuals are found in which the teeth are grossly overgrown due to damage. In a study carried out in Hamsterley Forest, Field Voles were found to ignore Wavy Hair Grass Deschampsia flexuosa. They were found to prefer grasses that had dead leaves etc. around their base (Gordon Simpson, pers. comm., 2012).
The voles tunnel into the litter and pull the more succulent grasses into their tunnel out of sight of aerial predators. Numbers of voles vary on two levels. There is a four-yearly cycle in numbers. Food and climatic conditions may affect numbers within this cycle but the real reasons for it are not fully understood. In the past, combinations of the cycle, good breeding seasons and ample food supply have produced plague years when vole numbers increased to enormous proportions, for example in south Scotland in 1892. As a result of this predator numbers also increase. During the “plague” years the voles can do a lot of damage causing a serious loss to agriculture. These plagues finish with a vole population crash, but the predator numbers may stay high for a year or so afterwards.
The presence of Field Voles can be indicated by the signs of feeding (small pieces of shredded grass etc.) and droppings, found in the surface runs. They are not readily caught during small mammal trapping: only eight of the 358 small mammals trapped by Veronica Carnell were Field Voles (V. Carnell, pers. comm., 2012).
Field Voles are common in the North East, but as with all small mammals our distribution maps do not show just how common. Our records tend to show the distribution of observers but the dense clusters of records in the boroughs of Darlington and Hartlepool are probably more representative of the situation across much of the North East. Nevertheless the distribution of records shows Field Voles to be at least present in almost all of the 10 km squares in the region. As with the Bank Vole, an indication of the numbers present can be obtained by examining owl pellets. The Field Vole is the preferred prey of Barn Owls Tyto alba and Short-eared Owls Asio flammeus and 1103 of the 1307 prey items identified from Long-eared Owl Asio otus pellets from Urlay Nook were Field Vole. If large numbers of shrew remains are found in pellets it is probably a sign that vole numbers are low.
Written by Don Griss (last updated Nov 12)