The Caledonian Forest is the UK stronghold for the agile red squirrel (Sciurus vulgaris), which has disappeared from most of southern Britain.

Global distribution

The Eurasian red squirrel is widely distributed in Europe and northern Asia, occurring from Scandinavia south to Italy and Bulgaria, and from Ireland and Britain across to Russia. From there its range extends all the way to Mongolia, China, Korea and Hokkaido, the northernmost of Japan's islands. The red squirrel lives both in conifer-dominated boreal forests and in broadleaved deciduous forests, particularly in western and southern Europe.

Distribution in Scotland

The red squirrel occurs throughout most of mainland Scotland, with the largest populations in the Highlands, in the Caledonian Forest remnants, and in Dumfries and Galloway. As a forest or tree-dwelling species, it is most common in the wooded parts of the country and is found in both native forests and plantations.

The population in Scotland has increased slightly in recent years, probably due to the expansion of tree cover, and is estimated at 120,000 individuals – 75% of the UK total. In contrast, the red squirrel has disappeared from most of its former range in England in the past 50 years. This is linked to the spread of the grey squirrel (Sciurus carolinensis), a non-native species introduced from North America in the late 19th century. The grey squirrel is better able to feed on broadleaved tree seeds such as acorns, and has displaced the red squirrel by competitive exclusion throughout much of England.

In Scotland, the red squirrel's range and population was also larger in the past, before the loss of most of the Caledonian Forest. For example, in 1994 on the virtually tree-less West Affric Estate, a pine cone was found, amongst the stumps of old Scots pines preserved in the peat, with the unmistakable signs of having been stripped by a squirrel (stumps such as those have been dated to be about 4,000 years old).

The red squirrel is classified on the World Conservation Union's 2003 Red List of Threatened Species as being Near Threatened, meaning that it is of conservation concern, but is not currently endangered. It is listed as a protected species on Appendix III of the Bern Convention for the Conservation of European Wildlife and Natural Habitats.

In the UK the red squirrel is protected by the Wildlife and Countryside Act of 1981, and is a Priority Species, and therefore the subject of a Biodiversity Action Plan (BAP), under the government's response to the 1992 Convention on Biological Diversity. A Scottish Squirrel Group, co-chaired by Scottish Natural Heritage and the Forestry Commission, was established in 1996 and a Scottish Strategy for Red Squirrel Conservation (PDF) was produced in 1998.

The red squirrel is a small mammal, measuring between 18 – 24 cm. in body length, and with a bushy tail up to 17.5 cm. long. Males are slightly larger than females and their weight averages 250 gm., with the largest individuals reaching 350 gm. There is considerable colour variation in the squirrel's fur throughout its geographic range, but in Scotland it is usually reddish-brown, except for its underside, which is white or cream. By late summer, the tail can be bleached to a pale or cream colour, prior to moulting. By contrast, the squirrel's body hair moults twice a year, once in the spring and again in the autumn. The winter coat is thicker and covers more of the soles of the feet, while the distinctive tufts of hair on the ears also become longer in winter.

The red squirrel is a member of the taxonomic order Rodentia, which is the largest group of mammals and includes mice, beavers and marmots. These are characterised by their front incisor teeth, which are specially adapted for gnawing and which grow throughout their lives. The squirrel's incisors grow at a rate of 15 cm. per year, but they stay short because of their continuous use and wear, particularly in opening pine cones and other seeds.

The eyes are large, dark and protuberant, giving the squirrel an excellent sense of vision, although it does not see in colour. The red squirrel's senses of smell, hearing and touch are also well-developed. It has long, sensitive whiskers on its muzzle, which it uses, together with special hairs on its feet and at the base of its tail, to aid its movement through the trees.

The squirrel's hind legs are longer than its forelegs and it has long claws and double-jointed ankles, making it very agile and well-adapted to its arboreal lifestyle. It is also able to squat on its haunches, freeing up its well-articulated front paws to hold and manoeuvre cones and other seeds.

The red squirrel's home is a drey, which is a hollow ball made of twigs and leaves in a fork amongst the branches close to the trunk of a tree. The inside is lined with moss, dried grass and soft hair, and a squirrel may have two or three dreys which it alternates between – it is thought this is to avoid the build up of fleas and lice. The drey is used for shelter and sleep at night, and also as a nest in which the young are raised. The red squirrel is diurnal, or active in the daytime, but in summer it will rest in the drey in the middle of the day. It does not hibernate, and relies on the ripe cones on pine trees and its stored food to survive the winter.

The red squirrel's main food consists of tree seeds, including acorns, hazelnuts, beech mast and, especially in the Caledonian Forest, Scots pine seeds. It has a special technique for quickly opening nuts, and it uses its incisors to strip the woody scales from pine cones to get the seeds they protect. Because of its light weight, it can reach cones growing at the tips of branches, and according to one estimate, a squirrel can eat the seeds from 20,000 cones in a year. It can also tell whether a nut is good or not by shaking it – a rattling sound indicates the kernel is shrivelled and therefore not worth eating.

The squirrel's diet includes buds, shoots, lichens, fungi, insects, berries and the eggs of birds such as the song thrush (Turdus philomelos), while it has also been observed peeling the bark off conifers, in order to lick the sap underneath. Hazelnuts, acorns and pine cones are cached under the soil surface for later retrieval, particularly during winter. While these nuts or seeds are being carried in the squirrel's mouth, chemicals from scent glands in its cheeks are transferred to the food, and these act as markers which help the squirrel relocate its hoarded supplies.

Reproduction begins as early as January, when one or more males will chase a female, making spectacular leaps through the forest canopy and spiralling up and down the trunks of trees. After mating, pregnancy lasts for 38 days and the young, or kittens, are born naked and blind, weighing about 28 gm. and 2.5 cm. in length. The litter averages three in size, although there can be as many as five young, and a female will have one or two litters a year, depending on the availability of food.

The kittens gain their body hair after 3 weeks and their eyes open after about 30 days. They are cared for solely by the female, and are weaned at 9 weeks. They gain their adult coats at 3-4 months, by which time they are fully independent. Females reach sexual maturity after one year, and individuals can live for six years in the wild, although many young squirrels do not survive their first winter if the conditions are harsh.

Although the red squirrel consumes large quantities of pine seeds, hazelnuts and acorns, it does not necessarily have a detrimental effect on the trees' ability to reproduce. A squirrel never recovers all the seeds which it caches, so some always survive to germinate and grow as new trees, often at a considerable distance from the parent. This is especially important for large-seeded trees such as oak and hazel, which rely on squirrels and other rodents to disperse their seeds.

Predators of the red squirrel include the pine marten (Martes martes), which is the only mammal able to chase and catch it in the trees. The red fox (Vulpes vulpes) and wildcat (Felis silvestris) will take a squirrel if they find one on the ground, and the stoat (Mustela erminea) will take young from the drey. The main predators however are raptors, including the golden eagle (Aquila chrysaetos), buzzard (Buteo buteo) and the northern goshawk (Accipiter gentilis).

The red squirrel is affected by a number of different parasites, including two species of lice (Enderleinellus nitzschi and Neohaematopinus sciuri) and by fleas (Monopsyllus sciurorum). It is also a host for ticks (Ixodes ricinus) and evidence from Switzerland implicates it, in some cases, as a transmission agent for the bacterium (Borrelia burgdorferi) which causes Lyme disease. Intestinal parasites, or coccidia, include several different species of protozoans (Cryptosporidium parvum Eimeria sciurorum and Eimeria andrewsi) and the squirrel is also susceptible to the parapox virus, which is fatal when contracted. Most squirrels, though, are healthy and the current measures to restore the Caledonian Forest are providing an increased habitat for these graceful arboreal mammals.