Squirrels

Red Squirrel Sciurus vulgaris

One of two species of squirrel in the area, the Red Squirrel is smaller than the Grey Squirrel, and has ear tufts (which Grey Squirrel never has). Colour is unfortunately variable but in summer at least it is predominantly red. In winter this can take on a greyish cast, but never with the mixed black, white and greyish hairs in the tail of the Grey Squirrel.
The Red Squirrel can still be found in much of rural Scotland and is still common in much of the north. It is often found in conifer woodlands, but also occasionally in broadleaved woods. (cf. Grey Squirrel, which is mainly an animal of broadleaved, especially parks and large gardens).
Delining rapidly in many parts of the UK, and extinct in much of the south. Reasons are not crystal clear, but in many urban places it has been replaced by the much larger, introduced American Grey Squirrel.
Evidence of Red Squirrels can be seen in the form of eaten pine cones, with all of the side "leaves" removed leaving a small tuft of them at the top. Although they are predominantly seed eaters squirrels will sometimes eat young chicks or eggs of birds in springtime, and will take insects too.
The "nest" is known as a drey and takes the form of a roofed-over bundle of sticks and twigs. They have been known to take over an owl nesting-box.

American Grey Squirrel Sciurus carolinensis

This introduced species is around 1/2 again as large as the Red Squirrel, and is now present in most of the cities around the UK, including Aberdeen in the north, where it can be found in public parks and larger wooded areas. Although the Grey sometimes has a reddish cast to the fur, it always has a grey tail, composed of black, white and reddish hairs, and never has ear tufts.

Mice

Wood Mouse Apodemus sylvaticus

The commoner of the two species of Mouse, and the only one of the small rodents which is found in houses all over the country, especially in early winter; though House Mouse may frequently be found in town houses.
Can be distinguished from the House Mouse by its larger ears, its general warm brown colour with white underparts, and usually has a yellowish spot on the chest. It has a longer tail than the House Mouse, generally about the same length as the body.
It can be found in most lowland habitats, woodland, fields and gardens.

House Mouse Mus musculus

Less common than the Wood mouse, and generally associated with towns and cities. It is a colder grey-brown colour, darker than the Wood Mouse, with pale grey underparts rather than pure white. The ears are smaller, and the tail shorter than the length of the body. It can be found anywhere where there are buildings, and can be a major pest of grain stores, bakeries, etc.

Voles

There are three species of vole in the area (although Orkney has a fourth species, an endemic sub-species of the continental Common Vole Microtus arvalis, which does not occur in the rest of Britain). Of these, Water Vole is easily distinguished being much larger (almost rat-sized) and usually very dark, sometimes black. The other two species are mouse-sized and rather tricky to tell apart; tail length is the best character.

Field Vole Microtus agrestis

Difficult to tell from Bank Vole in the field, but has smaller eyes and ears (sometimes ears barely visible through fur) and a shorter tail, which is dark above and pale below. Occurs all over Britain, and is probably the commonest prey species of Kestrel and owls. It can be found in field edges, sides of roadways, etc., constructing covered tunnels in long vegetation.

Bank Vole Clethrionomys glareolus

Difficult to tell from Field Vole in the field, but has larger eyes and ears (though still small) and a longer tail, which is uniformly coloured. This is more of a scrub and woodland species than the field vole, although can be found anywhere with even a tiny amount of woody growth, brambles, gorse, etc., everywhere across Britain.

Water Vole Arvicola terrestris

Associated with water courses, this vole makes its home in burrows on riverbanks. It is a large, black or dark brown vole, large specimens reaching the size of Brown Rat. In recent years it has suffered a great deal as a result of the introduction of American Mink, and is extinct in some parts of Scotland. It is a threatened species though still common in some parts.