The Spread of Grey Squirrels The grey squirrel was introduced into Britain in the late 19th century (1876 onwards), and has replaced the native red squirrel in most of its former British range. Red and grey squirrels occupy a similar ecological niche, as they are both small tree-dwelling mammals that rely on similar food sources (tree seeds, nuts, berries, etc). However, because of differences in their ability to digest large seeds from broadleaved trees (in particular, acorns), grey squirrels have a competitive advantage over red squirrels in broadleaved and mixed woodland, as they are able to feed on these seeds before they are fully ripe, and gain more nutritional benefit from them than red squirrels. This results in weight loss and reduced breeding success for reds. Grey squirrels are better adapted to living in broadleaved woodlands, which is why red squirrel conservation efforts are focused on large conifer forests, such as Ladybank and Devilla which are less attractive to grey squirrels.

Changes in Woodland Habitat
Between 1914 and 1945, there was extensive felling of woodlands throughout Britain. These, and subsequent felling of Scots pine and Norway spruce plantations as they mature, have largely been replaced with new plantations of Sitka spruce, that offer a limited and unpredictable food supply for red squirrels. Remaining woodlands have been further fragmented by the loss of an estimated 52.5% of the hedgerow habitat in Scotland between the 1940s and 1980s (SNH, 1997). Continuity of habitat is important, and the loss of hedgerow links between woodlands can result in the isolation of red squirrel populations. Felling can also lead to the isolation of red squirrel populations from both each other and their food sources, and increases the risk of losses from predation if red squirrels have to cross open areas.

Road Kills
A number of red squirrels are killed as the result of road accidents, particularly on smaller, less well-used country roads where there is woodland bordering either side of the road.

Disease
Outbreaks of disease can cause red squirrel populations to fall dramatically or even become locally extinct. The Squirrelpox virus, which is carried by grey squirrels but fatal if contracted by red squirrels, has been recorded recently (June 2005) in southern Scotland, an area otherwise free from the disease. This disease has been responsible for heavy losses of red squirrels in the north of England. Grey squirrel carrying the disease are making their way northwards from infected populations in northern Cumbria. This has serious implications not only for red squirrels in the southern Scotland, but Scotland as a whole.

Predation
Red squirrels that come to gardens to feed are vulnerable to predation from domestic cats, particularly if they have to cross open ground to get to feeding stations. Natural predators of red squirrels include pine martens and birds of prey, which may have a significant effect on local populations where squirrels become a favoured prey item. It should be noted that predators are just as likely to take grey squirrels as reds.