Finding and storing food is an import activity for squirrels and 70-90% of the time spent out of the drey is spent on these activities. Reds and Greys species feed primarily on seeds, which are high in energy, and squirrel numbers fluctuate according to the seed crop (more so for Reds than Greys). Plant matter, particularly berries and other fruit, also features prominently according to season. Ultimately, however, both species are opportunists and will readily take other foods if the opportunity arises.
A study published in Mammal Review in 1983, reported that, as well as eating seeds, Reds in a Scots Pine forest in eastern Scotland consumed fungi throughout the year with a peak (about 80% occurrence in the diet) between September and November. Conifer buds and bark were also consumed, primarily in winter and spring (December to May). The study also looked at the diet of Greys in English deciduous woodlands and found they fed primarily on deciduous seeds and fruit during winter and spring, after which the number of flowers and buds consumed increased, peaking in mid to late summer (July/August). Essentially, squirrels adapt their diet to take advantage of seasonal changes to the abundance of different foods and Greys do seem slightly more adaptable than Reds. Experiments by the BBC's Winterwatch series in 2015 found that Reds investigated a nut feeder put out in the forest within six seconds of arrive, while it took a Grey 12 seconds to pluck up courage to investigate. This was taken as evidence that Reds are less neophobic than Greys, although I would urge caution before drawing any such conclusion from such scant data.
More specifically, Red squirrels eat spruce and pine seeds, nuts (e.g. hazelnuts, beech, walnut and chestnut), acorn berries, fungus, flowers, shoots, pollen, bulbs, bark and sap tissue. Inclusion of soil and tree bark presumably contribute roughage and/or minerals. Grey squirrels will eat acorns and appear to thrive in oak woodland because they are more efficient at extracting the protein and energy from these seeds than Reds (see QA). Greys also take beech mast, tree shoots, flowers, pollen, bulbs, samaras (key fruit), nuts, fruit, roots, cereals and sap tissue.
|Tree Species||Energy per Cone/Seed (approx. KJ)|
|Oak||23.8 - 30 (species dependent)|
|Beech||2.8 - 5.5 (species dependent)|
|Hazel||18.7 - 25 (species dependent)|
Despite most species being far less nutritious than seeds, fungi are easily found and rich in nitrogen and minerals such as calcium, magnesium and potassium. Consequently, fungi is a particularly important food source for Red squirrels during the summer and autumn. The importance of fungi during the winter varies with habitat but and, in England and Finland, Reds devoted 90% of their active time during December looking for the energy-rich tree fungus Vuilleminia that grows under the bark of dead oak trees. In The Eurasian Red Squirrel, Stefan Bosch and Peter Lurz note that a variety of fungal species are eaten, including the “false truffles” Elaphomyces, Laccaria and Rhizopogon, the boletes Boletus and the deceiver mushrooms Laccaria. They also refer to a 2004 study that found, despite access to some 30 different species of fungi, Red squirrels showed distinct preferences, with just over half the fungi eaten being Rhizopogon species. Similarly, in Scottish forests Reds seem to show a distinct preference for ochre brittlegills (Russula claroflava). There is also the suggestion that certain fungi, some hypogeous species for example, give off an odour that attracts squirrels, although this has yet to be qualified. Fungi is often cached at the base of branches to allow it to dry before being eaten and analysis of the droppings of Red squirrels from Italy suggests they may be important dispersers of fungal spores.
Red squirrels will also feed on yew (Taxus baccata) seeds and berries. Between December 2012 and March 2014, a team of biologists from University College Cork, led by Amy Haigh, studied the feeding behaviour of Red squirrels living in a mixed broadleaf and conifer woodland on Fota Ireland, near the city of Cork in southern Ireland. They used a combination of radio-tracking and direct observations to look at the squirrels' movements and feeding preferences. The results, published in the journal Wildlife Research during 2015, show that the squirrels didn't use trees in accordance with their availability. Instead, they chose yew and beech most frequently and these species remained the preferred choice throughout the year. Between March 2012 and March 2013, for example, the team recorded 233 feeding observations.
Sycamore and oak were the most abundant trees in the study site, accounting for about 20% and 15% of the tree species, respectively; but they only accounted for only about 6% and 2% of foraging observations, respectively. Only about 14% of the trees in the study site were yew, however, yet Reds spent almost one-third of their foraging time eating yew seeds. Some 5% of the trees were beech but were the second most selected tree, accounting for just over 11% of foraging observations. Yew is generally considered to be a species preferred by Reds more than Greys, albeit that there are records of Greys eating yew and this is something I have witnessed myself, and that yew tree distribution may be an important consideration for Red squirrel reintroduction projects.
Both species are efficient at digesting high energy tree seeds and fruits and slightly less so the leaves and stems, although this may vary. In a 1979 paper to a Russian journal, I.P. Karpukhin reported that when seed crops in spruce forests of the former USSR were poor, Red squirrels switched to eating more vegetable matter, with high cellulose content. The result was that these animals developed longer intestines, with particular expansion of the large intestine and cecum.
Squirrels generally avoid seeds with high polyphenol content, because they disable enzymes in their guts and reduce their digestive efficiency and also reduce water absorption. Many acorns are high in the polyphenols called tannins and studies in America have shown that Greys will avoid trees the produce high-tannin acorns in favour of nearby ones with lower levels. Grey will also frequently cache acorns rather than eating them on the spot and it has been suggested that this may allow tannins to leach out of the acorn into the soil. Greys are, nonetheless, better able to process tannins than Reds and during their study on digestive efficiency in squirrels, published in 1993, Robert Kenward and Jessica Holm found that when given a mixed diet the Greys readily ate acorns, while the Reds ignored them. Only when given only acorns did the Reds feed on them and were about 40% less efficient at digesting them that Greys were, the researchers noting:
“Captive grey squirrels thrived on a diet of acorns, but red squirrels had a comparative digestive efficiency of only 59%, apparently because they were much less able than grey squirrels to neutralize acorn polyphenols.”
Indeed, the five squirrels fed on the acorn-only diet developed enteritis (inflammation of the gut) and died, despite the researchers switching them back to a mixed diet when they noticed the problem. Kenward and Holm also observed that the faeces of the acorn-only Reds were much looser than Greys on the same diet, containing about 40% more water and suggesting that dehydration may be a significant issue.
The range of many squirrel populations incorporates areas prone to seasonal flooding and this can restrict access to food, particularly to food caches. In a recent study of 42 Greys living in the Lowndes Wildlife Management Area, a patch of hardwood forested wetland in Alabama, Sarah Wilson and colleagues at Auburn University's School of Forestry and Wildlife Sciences found that, similar to Karpukhin's observations, during periods of flooding the squirrels increased the diversity of food taken, incorporating more herbaceous plants. Ten plant species were taken during winter flooding, including juniper (Juniperus sp.), chickweed (Stellaria spp.) garden pea (Pisum sativum) and trumpet vine (Campsis radicans). In their paper to Southeastern Naturalist during 2021, Wilson and her co-workers note that the squirrels didn't vacate the area in winter despite flooding rendering at least 70% of scatter-hoarded acorns unavailable. Instead they appeared to double the number of species in the diet during winter to compensate.
Both species will occasionally take insects, and recent footage captured on remote cameras by Stephanie O'Connor at Stirling University in 2016 clearly showed Greys predating the nests of ground-nesting bumblebees, during May 2016 and researchers at the University of Stirling have recently found that Greys will raid bee nests. There is also some evidence to suggest that they will take birds' eggs and chicks from nests, although data are lacking on how significant they are as avian predators. Sarah Dover described how, in late December 2017, two Red squirrel kittens visiting her garden on the Isle of Wight fought over the carcass of a dead bird (a robin, Sarah thought), before one picked it up and ran up a nearby tree. More recently, in late August 2022, Alison Mcwilliam reported seeing a Grey “squirrel carrying a wood pigeon, presumably dead. the squirrel was hopping along much like a weasel would with a rabbit” near Birmingham, although we don't know whether the squirrel killed the bird or found it dead. On YouTube, there are several videos of squirrels of various species eating small birds, including one of what appears to be a Grey taking a bird off a feeder and eating it, but I'm not aware of any other reports of them tackling birds the size of a pigeon. There is also one report I'm aware of describing a Grey removing a Red kitten from its nest, although the kitten's fate was undocumented, and a report of a Grey eating a live slowworm in a park near Waltham in Canterbury.
In urban environments both species will take food put out for birds, particularly peanuts, which can be problematic for them. Peanuts have a high fat content (about 44% fat, in fact) but are also very high in phosphate and low in calcium. This low calcium:phosphate ratio can retard calcium absorption in the gut and result in conditions such as osteodystrophy and metabolic bone disease. In other words, too many peanuts can cause thinning of the bones. In the The Eurasian Red Squirrel, Bosch and Lurz mention cases of bone thinning from Norfolk and the Isle of Wight; Helen Butler on the Isle of Wight and Susan Warn on Jersey have expressed similar concerns to me. Bone thinning caused by over- or prolonged consumption of peanuts may also make squirrels more susceptible to some injuries that appear feeder-related.
More recently, a peanut-heavy diet has been linked to changes in the masticatory biomechanics of Red squirrels. In a paper currently in preparation, Philip Cox, Philip Morris and Andrew Kitchener (University of York, University of South Florida and University of Edinburgh, respectively) present their analysis of the morphology of 250 squirrel jaws held in National Museums Scotland collection—specimens were compared from Scotland, England and Germany. Cox and his colleagues found that squirrels living at the Formby reserve in Merseyside, on the Lancashire coast, differed from Reds in the other populations in having a significantly lower mechanical advantage of the temporalis muscle.
The temporalis is a broad, fan-shaped slab of muscle situated on either side of the head; it's one of several muscles vertebrates use to chew and is particularly associated with the snapping shut of the jaw. The data collected by Cox and his co-workers imply that Reds at Formby are less efficient at gnawing than squirrels from elsewhere. Anatomical examination of the jaws found that the Formby squirrels had developed “a coronoid process that is positioned closer to the condyle and further from the teeth”. In other words, the muscles of the jaw attached further back; further away from the teeth and closer to the “hinge”, reducing the potential bite force. (Think of it like scissors—the closer to the hinge you put whatever you're trying to cut, the easier it is because greater pressure can be applied here than it can at the tips.) Cox and his colleagues suggest that supplementary feeding of squirrels at the Formby reserve with peanuts may account for the reduced temporalis muscle. Peanuts are much less mechanically resistant than most of the wild food items (acorns, beech nuts, hazelnuts, etc.) squirrels would normally take and the researchers conclude:
“Thus it is possible that the morphology of the mandible, and hence the efficiency of gnawing, has changed in response to this change in diet. This explanation is consistent with the results seen here, as the less mechanically demanding peanuts could lead to a reduction in the [mechanical advantage] of the temporalis.”
Bosch and Lurz suggest that any supplementary squirrel food should contain about 11% protein and 16% fat and in his Forestry Commission Research Information Note 235, published in November 1993, Henry Pepper laid out plans for a feeding unit that permits Reds but excludes Greys (based on weight) with, reference to the bait, considered:
“A suitable food mix is: 4 parts yellow maize, 4 parts wheat, 2 parts peanut and 1 part sunflower seed.”
Greys will also scavenge for leftovers of human food, and I have frequently seen them consuming the remains of discarded fast-food meals, crisps, cereal bars, discarded picnic food and even ice cream cones. Indeed, a highly adaptable diet is thought to be a key factor responsible for the rapid increase of Grey distribution. In particular, Greys are able to eat foods that are indigestible, to Reds, including unripe acorns—this is the so-called “phytotoxic explanation” for Grey squirrel spread and is covered in a separate QA.
Both species will also eat ash (Fraxinus excelsior) seeds and recent observations suggest that some Greys may even become “addicted” to them. Reader and friend Sharon Scott owns a smallholding in Buckinghamshire and, in 2016, noticed a lot of Grey squirrel activity in a small clump of ash trees on the farm. In particular, it is males found in these trees (of 23 individuals trapped in this area, 19 were males caught in ash trees) and one in particular was eating hundreds of the seeds in a day. Sharon told me:
“The ones that are eating the ash tree seed are very quiet, I can get close with no problems, while the others that aren't eating them scarper on the site of me!”
“I've never seen them on the ground, only in the trees and, as I've said, they are very laid back; almost doped!”
As far as I am aware, no studies have been conducted into the effect of ash seeds on squirrels and there are no data suggesting they contain anything specific that might “dope” them. Peter Lurz suggested to me that this might not be something that the tree itself produced, but might instead be the result of the ash seeds having been infected with a fungus that is producing compounds that are affecting the squirrels. Without chemical analysis of the seeds it is impossible to say, but it is an interesting observation, particularly given that a study by Luc Wauters and colleagues, published in 2002, observed that Grey squirrels at their study site avoided ash.
Grey squirrels consume between 40g and 80g (1.5 to 3 oz.) of food per day, which represents about 10% of their body weight, while Reds eat about 18g (0.6 oz.) per day, or roughly 5% of theirs.