Grey squirrel

Grey squirrels are an invasive species which were introduced to Britain from America in the early 20th century. Now, they cause conservation concerns for our native red squirrels.

Taxonomy chart

Animalia - Chordata - Mammalia - Rodentia - Sciuridae - Sciurus - S. carolinensis

Conservation status: UK Red List

GB: N/A 

England: N/A

Scotland: N/A

Wales: N/A

Global: Least Concern

Summary

Diet: Omnivorous. Tree seeds, tree flowers, shoots, fungi, bird eggs, and young birds. 

Habitat: Mixed woodlands, as well as urban and suburban environments. 

Size: Weight: 440-650g Size: 24-28.5cm with a 19.5-24cm tail. 

Lifecycle:  Females live up to 5 years and males 2-3 years. Females mature at 1. Breed from mid-winter to summer, having 2-3 kits a year, which are weaned at 10 weeks. 

Conservation concerns: None for itself. However, it outcompetes and spreads the deadly squirrel pox virus to the UK’s native red squirrels, causing conservation concerns.

Terminology

Drey: The nest of a squirrel, usually a spherical mass of twigs and leaves in a tree. 

Squirrel pox: A virus spread by grey squirrels to red squirrels, which grey squirrels can carry without contracting, but which is often fatal to red squirrels and can spread quickly through their populations. 

Caching: A behaviour squirrels undertake where they store food underground for the future. 

Immunocontraceptive: A drug which uses an animal’s immune system to stop it from reproducing. 

Geography 

Grey squirrels are found across most of Great Britain, except the Isle of Wight, the north-east of Wales, and the Highlands of Scotland. 

Grey squirrels favour deciduous and mixed woodlands, as well as urban and suburban environments. 

Biology 

Grey squirrels can grow to around 24-28.5cm, with a tail length of 19.5-24cm. Females can weigh 0.4-0.72kg, whilst males can weigh 0.44-0.65kg.  

Much larger than their red counterparts, grey squirrels host a silver-grey, agouti (speckled) coat, with a brownish tinge on their feet, face and along their backs, especially in summer, and a tail fringed with white. Sometimes, they can appear black, white, or albino, or very reddish, which leads to confusion with red squirrels. Grey squirrels are perfectly adapted for their arboreal existence, with a large, bushy tail that helps them balance, and double-jointed ankles that allow their feet to face either forwards or backwards when climbing. 

Female grey squirrels can live up to 5 years in the wild, and can breed from 10-12 months old, whilst males usually live 2-3. They can have two litters a year in early spring and summer. Young squirrels are born blind and hairless, in litters of 3-4, after a gestation of 44 days. Lactation lasts up to 10 weeks, though kits start to take solid food after about 8 weeks.

Ecology 

Grey squirrels are omnivores and enjoy a variety of foods. Their main food sources are tree seeds, such as oak, beech, hazel, sweet chestnut and walnut, but they also eat flowers, buds, shoots, pinecones, fungi, peanuts from bird feeders, birds’ eggs and even their young. To get through the winter, grey squirrels will often cache food underground during the autumn, and have even been recorded pretending to bury nuts whilst rivals are watching, in case they might steal their cache. Because deciduous fruit falls to the ground in autumn, grey squirrels spend much more time foraging on the ground than red squirrels. 

Grey squirrels are diurnal – active during the daytime – with peaks of activity in the early morning and late afternoon. They are not territorial but share home ranges and temporally abundant food sources. Grey squirrels typically make a nest of twigs, called a drey, which is cut live from the tree often with the leaves attached. Dreys are usually tucked in a fork against the trunk, though squirrels also use large holes in trees as drey sites. Grey squirrels’ main predators include pine martens, stoats, goshawks and foxes. 

Conservation 

Grey squirrels are a non-native, invasive species in the British Isles and thus the UK Red List does not apply to them. It has been illegal to keep grey squirrels (without a licence) since 1937, and it is illegal to release them into the wild. 

Grey squirrels are serious pests of forestry, as they strip the bark of thin-barked tree species, such as beech and sycamore, to access the nutritious sap beneath. This behaviour peaks in early summer, at about the time when the first litter of young become independent, and at the time when tree fruits are least available. It can kill the top of the tree or distort its growth. Grey squirrels can also be serious pests in gardens and among horticultural crops.  

Grey squirrels are also a threat to native red squirrels. They outcompete red squirrels for foraging opportunities and spread the squirrel pox virus, which they are mostly immune to, but which is fatal for red squirrels. This disease is the main cause of red squirrel decline across the British Isles as an outbreak can wipe out an entire population, causing lethargy, lack of coordination, ulcers, and swelling. 

To mitigate the spread of squirrel pox, grey squirrel management consists of shooting and sometimes trapping individuals. More recently, alternative management methods are being researched, such as a fertility control method, whereby grey squirrels are lured into feeding boxes to take food spiked with an immunocontraceptive, a squirrel pox vaccine for both red and grey squirrels, and a gene drive for grey squirrels, where genes would be modified to enable sex-biasing, and new grey squirrels would all be born male. Additionally, pine marten reintroductions have been shown to reduce grey squirrel numbers; grey squirrels, having not co-evolved with pine martens as a natural predator, are less fearful of them than red squirrels and thus more easily predated on. As grey squirrel management increases, red squirrel reintroductions can also take place to boost populations. 

History 

Grey squirrels have been present in the British Isles since between 1876-1929, when they were brought in from the USA by collectors and wealthy landowners. 

Grey squirrels, and other squirrel species, feature prominently in Native American folklore. In Timagami folklore, for example, it was said that the method of cooking squirrels had an influence on the weather – with boiled squirrels bringing rain. In Wabanaki folklore, it was said there was a squirrel called Mikew, who was a predator as large as a bear or wolf. Glooscap, a hero, thought he was too ferocious for humans and shrunk him to squirrel-size. Mikew remained ill-tempered and a troublemaker. 

After grey squirrels arrived in the early 20th century, they spread across the UK, bringing that trouble-making energy from Native American folklore to our native red squirrel populations, where they began outcompeting and displacing them due to spreading the squirrel pox virus. 

Identification and surveying 

Footprints: Tracks can be seen in mud, sand and snow. Their forefoot width is 2.5x3.5cm and hind foot width is 3.5x 4.5cm. 

Droppings: Droppings are usually found near trees. They can be dark grey or black and often sweet smelling, especially if the squirrels have been eating pinecones. They are small, round pellet shapes, 0.5-0.8cm long and 0.5cm thick. 

Feeding signs: Squirrels, much like birds, split nuts open at the top, leaving jagged edges and irregular pieces. They prise the nut apart after making an incision at the top. By comparison, mice and voles gnaw holes in nuts in a characteristic pattern. 

Nests: Dreys are spherical collections approximately 30cm across and at least 6m above the ground of twigs and leaves which are usually located in the fork of the branches, close to the trunk. It is easier to observe these in winter when there are fewer leaves on the trees. It is not possible to distinguish between red and grey squirrel dreys. 

Surveying period: Grey squirrels can be surveyed year-round as they are active all year, however, their dreys are more easily seen in the winter. 

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