Squirrels strip bark from trees primarily to reach the nutritious inner bark and cambium layer underneath, which they eat. The behavior is driven by a mix of dietary needs, seasonal food shortages, and population pressures, though researchers have spent decades trying to pin down the exact triggers and still do not have a complete answer. One of the more compelling recent explanations centers on calcium deficiency, but the picture is complicated by the fact that bark stripping varies wildly from year to year and from one woodland to the next.
What Squirrels Are Actually Eating
When a squirrel peels bark from a tree, it is not interested in the rough outer layer you see and touch. The target is the phloem and cambium tissue just beneath, sometimes called the inner bark or bast. This thin layer is where the tree transports sugars from its leaves down to its roots, making it a concentrated source of energy. Squirrel species worldwide are known to incorporate this inner bark and cambium tissue into their diets.1OSF Preprints. A Systematic Map of Research Exploring the Ecological Modifiers and Consequences of Bark Damaging Behaviour in Squirrel Species You can often spot the telltale signs: pale, exposed wood with small tooth marks running along its surface, and curled strips of bark scattered at the base of the trunk or along branches.
Sap flow also plays a role. During late spring and early summer, when trees are actively growing and sap is running freely, the inner bark is at its juiciest and most sugar-rich. Squirrels seem to time their stripping behavior accordingly. Early research suggested that variation in sap quality between trees could influence how extensively squirrels damage particular stands.2Mammal Review. The causes of damage by Red and Grey squirrels In other words, some trees taste better than others, and squirrels are selective about where they feed.
The Calcium Hypothesis
For years, the assumption was that bark stripping was simply about calories, but that explanation never fully added up. Squirrels have plenty of calorie-rich food sources available in most woodlands, especially during the seasons when bark stripping peaks. A more recent and increasingly discussed idea is the Calcium Hypothesis, which proposes that grey squirrels strip bark specifically to correct a calcium deficiency in their diet.3Forest Ecology and Management. A novel causal mechanism for grey squirrel bark stripping: The Calcium Hypothesis
Calcium is critical for squirrels. Their incisors grow continuously throughout their lives and are heavily mineralized, and females need substantial calcium stores during pregnancy and lactation. The hypothesis suggests that the cambium layer of certain trees provides a bioavailable source of calcium that squirrels cannot easily get from nuts and seeds alone. Supporting this idea, a controlled feeding study found that grey squirrels fed a diet where calcium was bound up as calcium oxalate, a form that is difficult to absorb, developed significantly lower bone mineral density than squirrels on a normal diet.4PubMed Central. Regulation of bone mineral density in the grey squirrel, Sciurus carolinensis: Bioavailability of calcium oxalate, and implications for bark stripping There was no measurable difference in bone density between the calcium oxalate group and squirrels fed a flat-out low-calcium diet, which implies that calcium locked in oxalate form is essentially useless to the animal.
If this hypothesis holds up broadly, it would explain some puzzling patterns. Bark stripping tends to intensify when squirrel populations are high and competing for food, precisely the conditions under which individuals are most likely to have marginal diets and mineral shortfalls. It would also explain why the behavior is more common in younger squirrels, whose skeletons are still growing and whose calcium demands are highest.
Food Shortage and Seasonal Patterns
Even before the calcium idea gained traction, food shortage was one of the leading explanations for bark stripping. The logic is straightforward: when the foods squirrels prefer, such as acorns, hazelnuts, and beech mast, are scarce, they turn to less-preferred alternatives like inner bark. This could be triggered by a poor mast year, where trees produce far fewer nuts than usual, or simply by the late-spring gap between the depletion of stored winter food and the arrival of fresh summer foods.
The timing of damage supports this. Bark stripping tends to peak in late spring through midsummer, roughly May through July in the Northern Hemisphere. That window aligns with both rising sap flow in trees and the period when squirrels are recovering from winter and rearing young. Research has noted that the behavior may well be triggered by food shortage, with the extent of damage then shaped by differences in sap quality between individual trees.2Mammal Review. The causes of damage by Red and Grey squirrels So the initial decision to strip bark could be driven by hunger, while which tree gets stripped might come down to taste or nutritional content.
There is also a social dimension. Some researchers have proposed that agonistic behavior, essentially aggression and territorial competition between squirrels, could trigger bark stripping. Under this view, bark stripping is partly a stress response or displacement activity, something squirrels do when population densities get uncomfortable. This idea has not been conclusively demonstrated, but it keeps surfacing in the literature and may work alongside dietary explanations rather than replacing them.
Why Population Density Matters So Much
One of the most reliable predictors of bark-stripping damage is not tree species, sap chemistry, or even food availability. It is how many squirrels are living in a given area, and specifically how many young squirrels are around. Surveys in Great Britain have consistently shown high variability between years in the damage recorded, with spikes in damage linked to high numbers of juveniles entering the population after a good breeding season.5PubMed. Changes in the impact and control of an invasive alien: the grey squirrel (Sciurus carolinensis) in Great Britain, as determined from regional surveys
This pattern has been documented repeatedly for grey squirrels. Earlier work found that the extent of bark stripping correlated with juvenile squirrel density, though interestingly only in years with relatively little squirrel breeding overall.6Journal of Zoology. Bark‐stripping by Grey squirrels That finding hints at a more complex relationship than a simple “more squirrels equals more damage” equation. In boom years when breeding is widespread, virtually all woodlands may experience some stripping. In moderate years, the damage may cluster in areas where juvenile density happens to be locally high.
Red squirrels show a similar relationship between density and damage, though they tend to cause less overall harm than grey squirrels. A study in a Scots pine plantation that had suffered considerable bark-stripping damage found fresh damage was related to local population densities, with the area supporting an unusually high density of about four red squirrels per hectare.7Mammal Review. Comparison of methods for obtaining population estimates for Red Squirrels in relation to damage due to bark stripping That is well above normal for red squirrels in managed forests, and it underscores the point that crowding drives the behavior regardless of species.
Which Trees Get Hit Hardest
Not all trees are equally vulnerable. Squirrels tend to favor certain species and certain age classes, and understanding these preferences can help you predict where damage is likely to show up. Surveys have found high variability between tree species in levels of damage recorded, but thin-barked species are consistently most at risk.5PubMed. Changes in the impact and control of an invasive alien: the grey squirrel (Sciurus carolinensis) in Great Britain, as determined from regional surveys That makes intuitive sense: thin bark is easier to peel and gives quicker access to the nutritious tissue underneath.
In British woodlands, beech and sycamore are particularly susceptible. Oak, hornbeam, and various conifers are also damaged but to varying degrees. Young to middle-aged trees, roughly ten to forty years old, tend to be targeted more often than very young saplings or mature timber. This is likely because trees in this age range have bark that is thick enough to reward the effort but still thin enough to strip without too much difficulty. Mature trees with deeply furrowed, rugged bark are harder to work with, while very young trees may not produce enough cambium to be worth the trouble.
The stand itself matters too. Trees growing in dense plantations or recently thinned woodlands seem to suffer more damage, possibly because these conditions affect both the squirrels’ food supply and the nutritional profile of the trees’ sap. Most observational studies have focused on identifying tree and stand characteristics associated with damage susceptibility, and that body of work makes up the bulk of what researchers know about the subject.8Mammal Review. Understanding Why Grey Squirrels (Sciurus carolinensis) Bark Strip in British Woodlands: A Systematic Map
What Bark Stripping Does to Trees
A tree that loses a patch of bark faces real problems. The cambium layer is the tree’s growth engine; it is where new wood and new bark cells are produced. When that layer is destroyed, the exposed wood dries out and becomes an entry point for fungal infections and wood-decay organisms. If stripping goes all the way around a branch or trunk, a condition called girdling, everything above that point dies because the flow of sugars from the canopy is completely cut off.
Even partial stripping causes lasting harm. The tree may survive but develops a wound that can take years to heal, leaving a visible scar and a weakened section of trunk. For timber producers, this is especially damaging. A single strip of missing bark on a commercially valuable tree can render the lower trunk useless for sawn timber, because the wound disrupts the grain and introduces discoloration. Multiply that by hundreds or thousands of trees in a plantation and the economic losses add up quickly.
Repeated stripping over multiple years compounds the problem. Trees that are hit year after year grow more slowly, are more vulnerable to windthrow because their trunks are structurally weakened, and are more likely to succumb to secondary infections. In severe cases, entire stands of young trees can be killed before they reach harvestable age.
Why This Problem Has Been So Hard to Solve
Foresters and wildlife managers have been trying to stop bark stripping for more than a century, and the honest answer is that nothing has worked well. A large-scale review of the research literature found that experimental studies aimed at preventing squirrel damage have come up empty: none identified an effective, practical, and humane solution for preventing or reducing bark stripping.8Mammal Review. Understanding Why Grey Squirrels (Sciurus carolinensis) Bark Strip in British Woodlands: A Systematic Map
The approaches that have been tried fall into a few broad categories. Physical barriers, like tree guards or wire mesh wraps, can protect individual high-value trees but are impractical at the scale of an entire forest. Chemical deterrents, sprayed onto bark to make it taste bad, have shown inconsistent results and need reapplication after rain. Supplementary feeding, the idea being to satisfy the dietary need so squirrels leave trees alone, has not reliably reduced damage and can actually attract more squirrels to the area. Population control through trapping or shooting can temporarily reduce local squirrel numbers but does not prevent recolonization from surrounding areas, and in the case of grey squirrels in Britain, the population is so large and widespread that local culls tend to be a temporary fix at best.
Earlier research suggested that habitat management might be a more promising long-term approach, potentially reducing the winter food supply for squirrels to keep populations in check while increasing food availability during the summer damage period.2Mammal Review. The causes of damage by Red and Grey squirrels But manipulating food availability across a landscape is complicated and slow, and the evidence that it works at scale is thin.
Grey Squirrels as an Invasive Species Problem
In Britain and parts of continental Europe, bark stripping is not just a nuisance. It is a significant ecological and economic issue tied to the introduction of the eastern grey squirrel from North America. Grey squirrels were released into British parks and estates in the late nineteenth and early twentieth centuries, and they have since spread across most of England and Wales, displacing the native red squirrel and causing extensive forestry damage.
Grey squirrels are generally more aggressive bark strippers than red squirrels. They are larger, live at higher densities in broadleaved woodlands, and produce more offspring per year, all of which translate to more trees being damaged. The spike in damage following good breeding years, documented across multiple surveys, is a persistent headache for forest managers.5PubMed. Changes in the impact and control of an invasive alien: the grey squirrel (Sciurus carolinensis) in Great Britain, as determined from regional surveys In their native range in North America, grey squirrels also strip bark, but the damage is less concentrated because forests are more diverse and squirrel populations are held in check by a wider array of predators.
The invasive-species angle is part of why so much of the research on bark stripping comes from the UK. It is where the economic and ecological costs are most visible, and where there is the strongest policy motivation to understand and address the behavior. Other invasive squirrel species cause similar problems elsewhere. Pallas’s squirrel, introduced to parts of Argentina, Japan, and Europe from Southeast Asia, strips bark from plantation trees with comparable results.
Bark as Nesting Material
Not all bark stripping is about food. Squirrels also peel bark to use as construction material for their nests, called dreys. The outer bark of certain trees can be shredded into flexible strips that squirrels weave into the structure of a drey alongside leaves and twigs.1OSF Preprints. A Systematic Map of Research Exploring the Ecological Modifiers and Consequences of Bark Damaging Behaviour in Squirrel Species This type of stripping tends to look different from feeding damage: it is often more superficial, focused on the outer bark rather than the cambium, and may occur higher up in the canopy where squirrels build their nests.
Nesting-related bark stripping does less harm to trees than feeding-related stripping because it usually does not penetrate to the cambium layer. The tree loses some protective outer bark but retains its growth tissue intact. That said, the two types of damage are not always easy to tell apart from the ground, and both contribute to the overall impact on a forest.
How Much Researchers Still Do Not Know
Given how long squirrels have been stripping bark and how much money and effort has gone into addressing it, it is striking how little is definitively understood about why they do it. A comprehensive review of the scientific literature found that evidence for the causes of bark-stripping behavior remains sparse, particularly for species that cause the most economic damage.8Mammal Review. Understanding Why Grey Squirrels (Sciurus carolinensis) Bark Strip in British Woodlands: A Systematic Map The vast majority of studies are observational, documenting where and when damage occurs rather than testing why it happens. Only about fifteen percent of published work has been experimental, and as noted above, those experiments have not produced effective solutions.
The Calcium Hypothesis is one of the few mechanistic explanations that has been tested in a controlled setting, and it showed promising results in laboratory conditions. But translating a feeding trial on captive squirrels into a field-level understanding of what drives wild populations to strip bark in particular places and particular years is a large step that has not yet been taken. Researchers still cannot reliably predict which woodlands will be hit in a given year, or explain why adjacent stands of seemingly identical trees can experience dramatically different levels of damage. Until those gaps are filled, managing the problem will remain more art than science, and the squirrels will keep peeling.