Although a familiar mammal in many parts of Great Britain, the grey squirrel (Sciurus carolinensis) is non-native, having been first introduced from the eastern USA in 1876. Introductions of this species to the UK continued up until 1915. Between 1930 and 1945 it underwent a huge expansion in range; it is now common throughout central and southern England, Wales and the central lowlands of Scotland and is still increasing in terms of range and numbers. Its native range extends throughout the eastern USA reaching as far north as Canada, and south to the Mississippi River.
The introduced species is larger than the native red squirrel (Sciurus vulgaris), has largely grey fur with touches of russet-brown and white underparts. The sexes are similar in appearance. Although very adaptable the grey squirrel prefers mature broadleaved woodlands with a rich understory layer. It also occurs in conifer woodlands, urban areas where there are mature trees, as well as gardens and parks. Active during the day they feed on seeds, nuts, buds, insects, bird eggs and fungi, depending on the time of year, and are well-known for their habit of hoarding food in autumn to see them through the harsh winter months. Seeds, cones or nuts are hidden in small scrapes scattered over the ground and buried. The general area is remembered, and then the cache is re-found by smell over fairly short distances.
Breeding takes place in December to February, and again in March to May. During this time, a number of males may follow a female when she is about to come into oestrus. During this ‘following phase’ the female may occasionally turn on the male and rebuff his advances by lunging at him aggressively. The day the female comes into oestrus, a number of males chase the female, making ‘buzzing’ noises; this is known as the ‘mating chase’, and the female can respond aggressively to males. Through much male-male chasing, dominant males are able to get closer to the female. When she is ready she crouches on the ground, and the first male to reach her mates with her. Gestation takes up to 44 days, during which time females are solitary and nest in a ‘drey’ of twigs and leaves. If conditions are good two litters are produced each year, consisting of 1-8 young. The young are usually weaned by 10 weeks and reach sexual maturity at 10-12 months of age. The average lifespan is 8-9 years.
This species is a serious pest in Britain, and its habit of removing tree bark is extremely damaging. In addition to out-competing red squirrels, it also carries a disease called parapoxvirus (PPV) which causes the fatal disease squirrelpox in the native species. Grey squirrels are actively controlled to protect trees and in areas where red squirrels persist.