In their 2012 Mammal Society book Squirrels, John Gurnell, Peter Lurz and Luc Wauters note that Red squirrel densities vary from 40 to 100 individuals per square kilometre, but there are no current census data for this species. In their 1995 review of Britain's mammal populations, Stephen Harris, Derek Yalden, Steph Wray and Pat Morris estimated that there were about 161,000 Red squirrels in the UK. Most Reds were to be found in Scotland (around 75%, or 121,000 animals) according to Harris and his co-workers, while 33,000 were in England and the remaining 10,000 in Wales. Populations of Reds have fluctuated considerably both historically and in more recent years. There are many records of Reds being imported from continental Europe, particularly Scandinavia during the 19th Century, to replenish populations when they ran low.
During the early 2000s there were also some significant declines, including along the Sefton coast in north-west England, where the population was believed to have crashed by 90%; at Formby, numbers had declined to an estimated 20 animals by 2008. More recently, however, populations appear to be increasing and there are now thought to be about 700 Reds on Anglesey (up from 300 around at the millennium). The Isle of Wight in Hampshire holds an estimated 3,500 Reds, while Brownsea Island in Dorset has around 250 and Kielder Forest in Northumberland is—by some estimates—home to 60-75% of England's Red squirrel population (almost 25,000 animals).
Overall, most squirrel charities appear to settle on a figure of between 140,000 and 160,000 Reds in Britain. Data on squirrel numbers in Ireland are scarce, although the final report of the Combined Research and Inventory of Squirrels in Irish Silviculture (CRISIS) summit, published in 2008, noted how: “there may be as few as 40,000 red squirrels remaining in Ireland”.
The Grey squirrel, by contrast, has thrived and expanded in both range and number. Gurnell, Lurz and Wauters, in their 2012 book, note that Greys can reach 800+ per sq-km in and, in their 1995 estimate, Harris and his colleagues suggest that there were about 2 million Greys in England, 200,000 in Scotland and a further 320,000 in Wales – a total of about 2.5 million in the UK. There have been no recent surveys to elucidate the current situation, but the number of squirrels killed by gamekeepers (so-called ‘gamebags’), which is often taken as a reflection of what is happening to a mammal population, doubled in Scotland and increased by 58% and 88% in England and Wales, respectively, between 1995 and 2009.
For many species gamebags can offer a misleading impression, because the number of a given species taken varies according to changes in effort, but these records do suggest and overall increase in the population since the 1995 estimate. The Breeding Birds Survey data also suggest that populations have increased, registering a 28% increase in overall abundance in the UK between 1995 and 2002. Indeed, I have seen some claims that Greys now outnumber Red 66:1 in Britain, which would imply some 10 million in the country. This statement is perhaps, however, a slight misinterpretation of a 2002 Forestry Commission press release in which they note that Greys were “estimated to outnumber” Reds 66:1 in England, which is closer to the 1995 estimate of 2 million-or-so.
Most recent references in the UK media suggest figures of between 3 and 5 million Greys in the UK, but none state their sources. The 2008 CRISIS report suggested there were in the region of 300,000 Grey squirrels in Ireland.