The wild forests of Scotland can be a challenging place in which to live, offering a generous dose of the elements: warmth-sapping wind, deep snow, freezing temperatures, driving rain and sometimes even blazing sun! Add to this the biting insects and hungry predators, and it is no wonder that many animals are quite selective about getting their ideal home just right.
However, for many creatures it is not a simple case of having a home in the way that you and I might think of one. A lot of animals have a territory, which is an area of land that provides their various needs such as food and breeding partners. This tends to include resting places and shelters, but these are often temporary or are used seasonally as breeding dens or winter quarters.
Nevertheless they are important. The many types of shelters and refuges animals use can be a major clue to their presence. They can also tell us quite a bit about the animals themselves and their role in the wider ecosystem.
Burrows and dens are two of the better-known kinds of animal home. Digging deep can be an effective way of keeping warm and dry, and of keeping predators at bay. The type of hole varies between species and can reveal an animal's identity. Badgers (Meles meles) are master excavators with powerful claws well adapted to the task of burrowing, whether for food or shelter. Unlike many animals their setts (as their burrows are called) are fairly permanent and can even be centuries old - these are true ancestral homes. Many of them are used all year round and can have extensive networks of tunnels and chambers. The entrances to badger setts are distinctive in that they tend to look like a letter D on its side. There may also be bits of dried grass or bracken (Pteridium aquilinum) outside which are among the materials used to make a warm, dry bed within the sett.
Fox (Vulpes vulpes) burrows or 'earths' have a more circular entrance and often have a distinct foxy smell about them. Interestingly it has been known for foxes to take up residence in badger setts, each species going about its own business!
The Scottish wildcat (Felis sylvestris sylvestris) is one of several mammals that make use of existing shelter such as caves, rocky overhangs or even large hollow trees, while the holt of an otter (Lutra lutra) will often be in a cavity beneath the overhanging roots of a riverside tree. The large carnivores, particularly the wolf (Canis lupus) and the lynx (Lynx lynx) (which have all been extirpated in Scotland), usually prefer dens where they have a good view over the surrounding landscape, enabling them to keep an eye on things. At the other end of the scale, small mammals such as mice and voles will dig tunnels or make use of small natural cavities.
The European beaver (Castor fiber) sometimes burrows into banks where conditions allow, otherwise their building prowess is demonstrated by the lodges they create. A beaver's lodge is a very robust structure made of sticks and mud, spacious inside and strong enough to keep out hungry wolves. For added security the entrance to the lodge is underwater, and they often keep a well-stocked larder inside to see them through icy spells. They may build a dam if the water is not deep enough for them, and sometimes the lodge is built into this impressive structure.
If you want to escape from ground predators, one option is to go up high, whether in trees or on cliffs. The most obvious tree-dwellers are the many bird species that live in the canopy. Their nests come in a fascinating array of styles and sizes. At one end of the scale are the moss-lined constructions of Europe's smallest bird, the goldcrest (Regulus regulus). At the other are the gigantic nests of birds of prey such as the golden eagle (Aquila chrysaetos) - massive platforms of sticks that may be added to year after year.
Among the mammals, the red squirrel (Sciurus vulgaris) is a well-known denizen of the tree tops. A squirrel's nest is known as a drey and is a ball of twigs and leaves around 30 cm. across, lined with softer material. Their summer dreys tend to be among the outer branches of the tree whereas in winter they build them close up to the trunk, keeping them safer from winter storms.
Insects and other invertebrates occupy a huge range of niches in trees and elsewhere. One example is the fissured, flaky bark of the Scots pine (Pinus sylvestris), which is an ideal refuge for a host of different insects and spiders. This also makes it a perfect hunting ground for birds such as the treecreeper (Certhia familiaris); its thin, curved beak probes the crevices and plucks the hapless bugs from their homes.
It has been said that there can be more life in a dead tree than in a living one. As surprising as this might sound, when we take a closer look, a dead tree can provide homes for a myriad of life forms. The great spotted woodpecker (Dendrocopos major) makes holes in dead trees as well as in living ones weakened by disease. The crested tit (Parus cristatus) is a pinewood specialist that also nests in holes it excavates in dead trees of a certain diameter, while bats often roost in cavities in dead or old trees. Such trees are something of a wild metropolis!
Now let us zoom in on the invertebrates. Many beetles and flies love dead wood. Some specialists such as the longhorn beetles (Family: Cerambycidae) are among the longest-lived insects. Their larvae spend many years sheltered deep within the wood, growing very slowly on a low-nutrient diet, and relatively safe from predators (except for, perhaps, the occasional hungry woodpecker or bear!). Often the only sign we see of them are the exit holes left by the emerging adults. Others, such as the bark beetles, prefer the more nutritious cambium, just beneath the bark of living trees. The bark provides the larvae with protection while they fatten up before emerging.
Along with dead trees, ancient or veteran trees are also an incredibly rich habitat. They often have a lot of dead wood in their canopies, and even rotting, hollow trunks that are used as shelter by all sorts of creatures.
Not all animals are so demanding in their requirements. Foxes and many other animals use burrows at certain times of the year, but at others times may simply rest under dense vegetation, or in a secluded, sunny spot. Deer tend to just lie on the ground. Roe deer (Capreolus capreolus) clear away debris and vegetation, but red deer (Cervus elaphus) don't even do that. However even these hardy beasts often make use of natural shelter, and in winter when cold winds and rain are the death of many deer, they take shelter in fragments of forest. (In the past red deer were much more of a forest animal, but loss of this habitat has forced them into more open areas). In contrast, summer brings midges (Culicoides impunctatus), which can become so hard to bear that red deer often move up to higher ground where the breeze keeps these biting insects away.
Another ground dweller is the wild boar (Sus scrofa), which tends to construct simple, temporary nests made from surrounding vegetation. Without the refuge of burrows or trees, animals such as deer and boar have to rely more on their ability to fight or flee if confronted with a predator.
Some birds such as the capercaillie (Tetrao urogallus) are ground nesters. Ground nesting birds are more vulnerable to predators, and have to compensate by having eggs that blend into their surroundings. The hen bird also relies heavily on camouflage to compensate for her exposed position.
The world of invertebrates can really highlight the incredible diversity of potential dwelling places. Take for example wood ants (Formica spp.), which build perhaps the most elaborate home in the forest. Their miniature cities are a great example of eco-design: south-facing to provide passive solar heating, and thatched with pine needles to shed the plentiful rain! Within the nest there are numerous tunnels and chambers in which the temperature is carefully regulated to ensure the best growing conditions for eggs and larvae.
Wasps are quite closely related to ants and like their cousins they can be very social creatures. Some species, such as the common wasp (Vespula vulgaris) build astonishing nests. For a start they are beautifully geometric with hexagonal chambers within which their larvae develop. Hexagons form a particularly strong structure, and they are actually the most efficient shape for creating adjoining cells with minimum use of materials. This strength and efficiency are both important considering that their nests are made of paper! The wasps chew dead wood to create the pulp, and make numerous journeys to complete their home. If you are near a bit of dead wood or an old fence post on a summer's day, listen carefully and you might hear the quiet rasping of a busy wasp gathering building material.
Some homes are simply bizarre and could make great material for science fiction! Many people are familiar with 'cuckoo spit', the small blobs of frothy liquid that are often seen on plants in the summer. These are the homes of the froghopper (Philaenus spumarius), a tiny bug that creates the froth from a sugary substance secreted from glands in its abdomen. And few homes are more bizarre than galls: strange growths on plants triggered by the invasion of various organisms, including certain insects. Galls come in an array of shapes and sizes, and the shape of the gall, along with the plant it is on, can reveal the identity of the inhabitant. Spangle galls, for example, look like tiny flying saucers attached to the undersides of oak leaves (Quercus spp). In early spring a tiny wasp known as Neuroterus quercusbaccarum lays its eggs in the oak buds. Remarkably this stimulates a response in the plant that creates the ideal structure for the developing grub, offering both food and shelter.
As good residences are at such a premium, an abandoned nook or cranny will rarely be empty for long. Once vacated, woodpecker holes may quickly be occupied by any of several birds for nesting. Sometimes whole groups of the diminutive wren (Troglodytes troglodytes) will huddle together in an old woodpecker hole for warmth.
Osprey nests are so large that it is not unknown for other, smaller birds to build their own nests within the structure while the ospreys are still present! Some nest occupants are less welcome: parasites such as fleas take up residence alongside their host birds, many of which have to change their nest site each year to avoid the onslaughts of their bloodthirsty neighbours.
Some animal homes can have a noticeable influence on the surrounding ecosystem. The regular deposits of droppings around badger and fox dens can enrich the soil and encourage more nutrient demanding plants such as nettles (Urtica dioica) to thrive, and they in turn are a foodplant for caterpillars of the peacock butterfly (Inachis io), which gain protection from predators while they pupate by the stinging hairs on the nettles. The presence of a wolf den can deter deer from feeding in an area, and so lead to healthy thickets of trees and shrubs. This in turn provides nesting sites for birds and other wildlife. Beaver activity can provide homes for many other creatures. For example, by flooding a patch of forest they may create valuable standing dead wood.
Without the right kind of home many animals will simply vanish from an area. Providing artificial accommodation is an effective way of helping certain species of wildlife to thrive when their habitats have been damaged. Wildlife gardeners in urban areas are well aware of the difference something as simple as a bird box can make. Even in some wilder areas, wildlife is so impoverished, or certain kinds of refuge are so scarce, that a bit of human assistance can have a significant effect. As we know, dead trees are a particularly valuable form of shelter, but they are in short supply in many forests, due to intensive management. Providing specialised bird and bat boxes can simulate the holes that many species require. Boxes for crested tits are filled with sawdust to simulate the soft dead wood they are accustomed to excavating!
Obviously these are important but short-term measures and only benefit certain species. While such techniques are necessary, in the longer term the best strategy is to restore plenty of wild, varied habitat, to ensure that there will be an abundance of homes and refuges for a wide range of wildlife.
Sources and further reading
Nest cells support heavy weights: bees and wasps http://www.asknature.org/strategy/735f37e668dea62b70f763f6ca154bba
Bang, P. and Dahlstrom, P. (2001) Animal Tracks and Signs. Oxford University Press: Oxford.
Brown, R.W., Lawrence, M.J. & Pope, J. (2004) Animals - Tracks, Trails and Signs. Hamlyn: London.
Chinery, M. (2005) Complete Guide to British Insects. Collins: London.