The Mammals of Deciduous Woods
Although deciduous woods can seem to be uninhabited, tucked away in the different layers of vegetation are many of our mammals, from large badgers and foxes to small mice, shrews and voles. Each has a special niche within this rich environment.
Deciduous woodland supports a greater variety of mammals than any other natural habitat. Many of today’s woodlands are remnants of the great broadleaved forest that once stretched across the whole of northern Europe. Two thirds of our deciduous woodland is either native beech or oak, usually combined with species such as birch, ash, hazel, sycamore and various shrubs.
Deciduous woodlands consist of four main layers of vegetation, each providing living space for different species. Some, such as common and pygmy shrews, spend their entire lives at one level; others, such as squirrels and woodmice move between the layers. Competition for space is not as great as it is in the less complex habitats of fields or moorlands, and many more species are accommodated into the woodland environment.
The highest point is the tree or canopy layer, comprising the branches of mature trees from 6- 18m (20-60ft) high or more. Young trees, shrubs and bushes form the next level, the shrub layer, a dense growth sometimes called the lower canopy. Here such species as blackthorn, buckthorn, dog rose, elder, hawthorn and spindle flourish. Below is the field layer which extends to about 2m (6ft) above ground level and consists of low woody and herbaceous plants, flowers, ferns and mosses. This is the richest level in the variety of wildlife it supports, and most woodland mammals live or feed here. Below is the litter and soil layer.
Most mammals are more dependent on the general structure of deciduous woodland than on any individual plant species contained there. Important factors are the height of the vegetation, the presence or absence of field and shrub layers, and whether there are areas of open land, scrub and old trees. The bank vole, for example, is most common in woodlands with dense herbaceous areas, while the woodmouse is the common rodent of woodlands with an open field layer.
This layer attracts mainly insects and birds. Oak trees, in particular, are noted for their rich insect life-well over 200 species may live on a single tree during its life-span of up to 250 years. This activity not only draws numerous birds, but some mammals spend much of their lives high in the branches. Several species of bat hunt around the tops of trees for flying insects and some, notably the long-eared bat, specialise in picking insects off the foliage. Bats often inhabit holes in the upper reaches of the trunk. In beech woodland during the day you can sometimes hear the noctule bat chattering in a tree hole, and nursery colonies are also found in tree holes.
The most common mammal of the tree canopy of deciduous woodlands is the grey squirrel. It thrives particularly in oak and sweet chestnut woodlands, feeding on nuts, bark (especially that of sycamore), shoots, fungi and birds’ eggs. In winter it is quite easy to spot its untidy spherical drey of twigs and dead leaves high in the branches, or at the junction of a branch and the main trunk.
A mammal sometimes confused with the grey squirrel is the fat dormouse, also known as the grey, edible or squirrel-tailed dormouse. It is confined to an area around Tring in the Chilterns, where it was first introduced earlier this century. However, it is smaller than the squirrel and entirely nocturnal, spending its day in a nest built close to the trunk of a tree or in a tree hole. It has a similar diet to the grey squirrel, and can cause damage to trees. Both species are accomplished acrobats, making great leaps between branches, using their tails to help with balance.
The red squirrel has lived in the British Isles for over 9000 years, particularly in the native Scots pine forests of the north. Today it lives mainly in pine woods, but in some parts of the British Isles it has adapted to deciduous woodland, above all to areas containing hazel and oak. Because of its arboreal nature, it has few predators, although it is occasionally pursued by the pine marten in areas of mixed natural woodland in the Scottish Highlands. In general the red squirrel population is declining in deciduous woodlands, although in some areas it co-exists well with the grey squirrel.
This layer consists of the lower branches of mature trees, saplings and shrubs. And it is often entwined with such climbers as ivy or honeysuckle. Indeed, stripped honey-suckle stems are a sign of the presence of the common, or hazel, dormouse, an increasingly rare mammal which lives mainly in the shrub layer, although it also climbs into the canopy. It prefers woodlands with plenty of dense secondary growth and scrub, where hazel, beech and sweet chestnut are the dominant tree species. It makes its daytime roosting nest in the undergrowth or in the cleft of a sapling.
The dormouse is a solitary, nocturnal animal and one of the best times to look out for it is the twilight of late summer and early autumn evenings when it emerges from its daytime sleep to eat all the fruits and nuts it can find so as to gain weight before its long winter hibernation from October to April. One sign of its presence is discarded hazel nuts with smooth, neat, round holes; other rodents leave a more ragged hole.
Deer are creatures of the shrub layer. They remain hidden during most of the day, moving from cover in the evening to graze, and returning at dawn. In undisturbed areas they may be active during the day, but usually all you will see is a disappearing rump as the animal runs off into the trees.
Sika deer have established feral herds in Dorset, Hampshire, West Yorkshire and parts of Scotland. They prefer deciduous or mixed woodland with an undergrowth of hazel, bramble, blackthorn or other shrubs. Sika are grazers, feeding mainly on grasses and sedges in fields and open land within easy distance of protective cover.
Roe deer are mainly browsers, taking leaves, branches, shoots and bark of broad-leaved trees including ash, hazel and oak. In many areas they are considered a serious pest. Fallow deer and muntjac are both grazers and browsers, depending on the availability of food. Fallow deer have the widest distribution of all the deer species in England, while muntjac are mainly confined to the south-east.
Dwelling in the field layer and at ground level are woodmice, bank voles, shrews, hedgehogs, rabbits and hares, and their predators the fox, stoat, weasel and, in Wales, the polecat. The badger, largest of the woodland mammals, eats small mammals occasionally but its main diet includes amphibians, slugs, snails, grubs, insects and earthworms (probably its most important food item), as well as fruits, roots, tubers and nuts.
Some of these species, such as shrews and bank voles, are active day and night. The common shrew makes runways and tunnels through the litter and soil and can sometimes be heard making a soft twittering sound as it searches for invertebrates to satisfy its voracious appetite. It eats at a furious rate for two hours or so, then rests, and then eats again, for it must take in its own body weight each day. The pygmy shrew also lives in woodlands, but it is less abundant than its relatives. The bank vole builds a nest of grass and moss under roots and in tree stumps. It is an active burrower. But most of its food is obtained from the field layer.
Moles are also both nocturnal and diurnal, although mole hills are likely to be the only evidence of their existence, for adult moles are seldom seen above ground. The mole digs its complex tunnel system underground, pushing the loose soil upwards to form the characteristic molehill. People do not always realise that deciduous woodlands have abundant mole populations because the large nest mound or ‘fortress’ is usually built in dense undergrowth.
Of the woodland’s mammal predators, the stoat and weasel are the most active during the day. They are mainly ground-level hunters, following regularly revised paths and feeding on locally available small to medium sized mammals such as mice, voles and rabbits.
Towards dusk, when truly diurnal mammals such as the grey squirrel are eating their last meal of the day, a night-shift begins to appear. Bats, including the noctule, pipistrelle, Bechstein’s and serotine, emerge from their roosts in tree hollows or beneath foliage in the upper canopy, and can be heard squeaking and chattering as they chase insects, including moths.
Around dusk the badger emerges from its sett. It has no predators apart from man, and its conspicuous colouring serves as a recognition sign for other badgers at night, although it appears grey from a distance. Its ideal habitat is a beech wood on light, well-drained gravelly soil, with pasture and arable land within its home range, although it is well adapted to all types of deciduous woodland. Another carnivore, the fox, is one of the most successful of all our mammals, managing to live in almost all types of habitat in the British Isles. In woodlands its diet includes small rodents, rabbits, hares, birds, insects and carrion.
The most nocturnal mammals are the hedgehog, fat and common dormouse, wood-mouse and yellow-necked mouse. The hedge-hog sleeps in undergrowth during the day, changing its site frequently during summer. It searches at night for ground level invertebrates.
The deciduous woodland in autumn provides a rich supply of food for its inhabitants. Nuts and pulpy fruits, as well as fungi, are eagerly seized. Even the highly carnivorous fox takes its share. To some species the autumn harvest is of special long-term importance. The grey squirrel stores surplus nuts in the ground, in tree hollows and in dreys. In times of shortage it can remember the general area where the caches lie, but damp conditions are needed before it can locate, by its sense of smell, the exact position of a hoard. Many of these caches remain unused or undiscovered, and the seeds and nuts may germinate to become new trees. In years of poor mast harvest many mammals, especially those with specialised or restricted diets, such as the edible dormouse, die before their first winter. Many young mammals leave home in autumn, and the available food supply from trees and shrubs can be a vital factor in their survival. Many mammals reduce their activity, but in the British Isles only a few truly hibernate: hedgehogs, bats and the two species of dormouse. The metabolism slows right down, the heart beats much less often, the animal breathes more slowly, and its temperature drops. In this way it conserves its energy. Warm days in mid-winter can be disastrous for hibernating animals, for their body temperature heats up and they use their vital stored energy too fast, with no means of replacing it until spring.
For the deer on the other hand, autumn and winter are the most strenuous period of the year. For the fallow deer, in particular, autumn is the time of the ‘rut’, a period of tremendous activity for the males, which mark out their territories by fraying trees and shrubs with their antlers and by scraping the leaf litter and soil. Rutting areas, which are usually beneath trees, can be recognised in autumn by the well-trodden paths around the bases and by the musky scent of the bucks.