Invariably the failure of seed crops and subsequent starvation is the single biggest source of mortality for squirrels, but they do also fall prey to a range of avian and mammalian predators in the UK and throughout much of Europe. (It should be noted that most records of squirrel in the diet of a predator come from stomach or faecal analysis, which means we cannot know whether the squirrel was caught and killed by the predator in question, or whether a dead animal was simply scavenged.)
Squirrel remains have been found in the pellets of several bird of prey species, including long-eared owls (Asio otis), tawny owls (Strix aluco) and in Europe, eagle owls (Bubo bubo) and Ural owls (Strix uralensis). Remains have also been recorded from the diet of red (Milvus milvus) and black kites (M. migrans), peregrine falcons (Falco peregrinus) and sparrowhawks (Accipiter nisus). During his studies on the natural history of buzzards (Buteo buteo) in the New Forest, starting in 1962 running until his untimely death in the autumn of 1997, Colin Tubbs observed grey squirrels being taken as prey, particularly during years when small rodents (i.e., rats, mice and voles) were scarce.
In recent years, there has been a growing body of evidence that buzzards are also taking Red squirrels on the Isle of Wight and concerns have been raised by islanders that this might send the squirrel population into decline. There is currently no indication that buzzards are a significant predator of squirrels on the island, or that the island's squirrel population is declining. Reds do, nonetheless, appear to recognise buzzards as predators. In February 2011, wildlife photographer and amateur naturalist Roger Powely was watching a squirrel feed on a tree stump at Alverstone on the Island. Mr Powley told me:
“All of a sudden a buzzard started calling up above the trees. The squirrel immediately froze and didn't move again until the calling had stopped.”
More recently, an escaped red-tailed hawk (Buteo jamaicensis) was photographed feeding on a Red squirrel on the Isle of Wight. An article in the island's County Press newspaper in November 2017, reported that the raptor “has been blamed for a sharp decline in red squirrels in countryside between Ryde and Seaview” after local residents reported fewer squirrels visiting their gardens immediately after the hawk escaped.
In terms of raptors, it is generally accepted that the northern goshawk (A. gentilis) is probably the most significant. In The Eurasian Red Squirrel, Bosch and Lurz refer to a comprehensive study of bird diet that recorded a total of 268 Red squirrel remains, 235 (88%) of which were taken by goshawks. Several authors have noted how goshawks tend to predate squirrels more often during the winter months, presumably when they're easier to spot among the bare branches and other prey may be scarce. A study of the diet of goshawks in a boreal forest in central Sweden between 1977 and 1981 by Per Widén found that squirrel dominated prey remains, both in terms of number and weight, during the winter. In a paper to the journal Holarctic Ecology published in 1987, Widén noted:
“The most striking dietary difference between breeding season and winter was in the proportion of squirrel. In winter, it amounted to 79% of the prey animals taken, in spring and summer only to 14%.”
Steve Petty, Peter Lurz and Steve Rushton investigated the potential impact of goshawks on Red squirrels in Kielder Forest, Northumberland. They collected prey remains from active goshawk nests in the study area between 1973 and 1996 and estimated the squirrel population. When goshawk numbers were low, between 1973 and 1979, no squirrel remains were found in the diet. Between 1980 and 1996, the remains of 97 Red squirrels were identified, accounting for just under 2% of the goshawks' diet. Curiously, they found that more squirrels were taken during the breeding season (March to August) than at other times of the year and, consequently, that the number taken during the winter was comparatively low. On average, they estimated that each goshawk in the study area took about four squirrels per breeding season. When they used this estimate in their model, the suggestion was that about 79 squirrels were killed by goshawks in Kielder each year, although the authors acknowledge that the proportion of squirrel biomass in the diet varied to such an extent from year to year, depending which year's estimate you used the number of squirrels taken ranged from eight to 261 per year.
In order suppress the growth of the Red squirrel population in Kielder, Petty and his colleagues considered that the goshawks would need to take at least half the population each year. Even based on the lowest population estimate they arrived at, this would require taking just over 1,700 squirrels per year and even the most extreme estimate of predation doesn't come close to this. In their 2003 paper to the journal Biological Conservation, the biologists concluded:
“This conclusion is in broad agreement with other studies indicating that food availability (conifer seed) is the main factor limiting numbers of tree squirrels, not predation.”
In a study of goshawk prey remains collected during the breeding season (March-September) from an area of mixed forest, farmland and moorland in Wales between 1991 and 1993, Paul Toyne found that Grey squirrels accounted for just under 8% of the prey items. Toyne also found that Grey squirrels were more common in the diet of goshawks nesting in small forests than large ones in his study area. More recently, observations of goshawks around nests in Devon and Derbyshire revealed 68% and 95%, respectively, of the prey they were taking were Greys, while 90% of the diet of a pair nesting in the Midlands during 2014 was Grey squirrel. In Finland, a team of biologists at the University of Turku led by Vesa Selonen found that goshawks were the main cause of mortality in radio-tracked squirrels and made up 1-2% of the diet of Ural owls during the summer. In a paper to Annales Zoologici Fennici during 2016, Selonen and her colleagues presented data suggesting that the presence of nesting goshawks and Ural owls significantly reduced Red squirrel activity around feeding sites during the winter.
Birds of prey aren't the only avian predators of squirrels. In her Red Squirrels on the Isle of Wight booklet, Helen Butler writes of a magpie (Pica pica) puncturing the chest of a Red squirrel with its beak, and a crow (Corvus corone) breaking the neck of another. More recently, parakeets have been observed attacking or predating squirrels. In their 2013 paper to Hystrix, Emiliano Mori and colleagues described a female Barraband's parakeet (Polytelis swainsonii) attacking a Red squirrel near Follonica in Tuscany (Italy) during July 2012:
“The parrot repeatedly pecked at the head and the back of the squirrel, accompanying the attack with loud alarm calls. The squirrel tried to defend itself by covering its back with the tail and moving along the branch, but after a few seconds, it fell dead to the ground.”
Additionally, in her engaging book Wild City, published in 2022, Florence Wilkinson recounts an interview with Matthew Frith, London Wildlife Trust's Director of Conservation, in which they discuss the spread of ring-necked parakeets (Psittacula krameri) in the capital:
“A couple of years ago we were taking some people on a walk and we saw [a parakeet] fly up into a hole in an oak tree, yank out two baby [Grey] squirrels and break their necks...”
Matthew subsequently told me that this happened in a stand of semi-mature oak trees in Sydenham Hill Wood during October 2017. The parakeet took kittens out of the nest, dropped them about six metres (20 ft.) to the ground and showed no further interest in them. No adult squirrel appeared during the time they were watching.
The list of mammalian predators that will take squirrels if the opportunity arises is considerable and includes red foxes (Vulpes vulpes), domestic cats (Felis catus), wild cats (Felis sylvestris), domestic dogs (Canis familiaris), badgers (Meles meles), polecats (Mustela putorius), weasels (M. nivalis) and mink (M. vison). Stoats will also predate squirrels and Sarah Hibbett filmed just such a chase, apparently ending in the stoat going hungry, in a forest in Sandringham, Norfolk during late December 2016.
The pine marten connection
In Europe, the pine marten (Martes martes) is probably the squirrel's most significant mammalian predator. Pine martens are at home in the treetops and will actively chase squirrels through the canopy. As with goshawks, however, there is considerable variation in the reported prevalence of squirrel in the diet of martens, and they most often feature on the menu during the winter. Indeed, observations by Craig Shuttleworth suggest that while pine martens may kill a number of Grey squirrels, they frequently do not eat them, and Shuttleworth suggested to me this is because Greys are actually very tough and difficult to skin.
Dietary analysis of martens in Scotland suggests they do not prey on Red squirrels here, and William Paterson and Gavin Skipper failed to find any evidence of squirrels in the diet of martens at four sites in the Loch Lomond and The Trossachs National Park in Scotland. By contrast, a study by Erkii Pulliainen and Paivi Ollinmaki found that in some winters Red squirrels made up 11.5% of the diet of the pine martens in a boreal forest in Finland. Similar studies in Sweden have suggested that pine martens there may switch to squirrels when vole numbers run low.
A study of pine marten diet in Ireland led by Emma Sheehy at the National University of Ireland in Galway and published in the European Journal of Wildlife Research during 2014 provided anecdotal evidence that pine martens may preferentially predate Grey squirrels over Reds, but confirmation is required. Some biologists suggest any apparent relationship may simply reflect the Red squirrel's ability to venture onto thinner branches than Greys and therefore evade martens more effectively, although this seems unlikely as a sole explanation and it may have more to do with the Red's flighty nature. In a paper to the journal Biological Conservation in the same year, Emma Sheehy and Colin Lawton presented data on the relationship between Grey, Red and marten populations in the midlands of Ireland. Their data suggest that the Grey population had collapsed by 2012, and the abundance of Red squirrels and pine martens had increased in the same timeframe. Furthermore, Sheehy and Lawton note that:
“At landscape level, pine marten and red squirrel abundance were positively correlated, whereas a strong negative correlation between pine marten and grey squirrel presence at woodland level was found to exist.”
So, although more work is required on this, these data seem to suggest that pine martens may have a role to play in the reduction of the Grey squirrel population and the resurgence of the Red in Britain.
In North America, snakes, raccoons (Procyon lotor), red foxes, grey foxes (Urocyon cinereoargenteus), bobcats (Lynx rufus) and coyotes (Canis latrans) will take Grey squirrels.