Grey squirrels have long been the bane of foresters’ lives. In our latest Voices of Forestry column, Theresa Reichlin summarises the findings of her 2017 master’s thesis, which came to some interesting conclusions about why the rodents love tree bark so much. 

GREY squirrels (Sciurus carolinensis), invasive to the UK, pose a significant threat to native red squirrels and the British forestry industry. While previous studies have discussed their impact, there’s limited emphasis on how social learning (culture) influences behaviour patterns like tree bark-stripping. In my capstone project from my Master of Forest Conservation programme at the University of Toronto, Canada, I explored this with the help of my professors, and my colleagues in the UK who were very inspirational to me during my Prince of Wales Forest Leadership Award internship exchange in 2017.


Originating from North America, grey squirrels were introduced to the UK centuries ago, thriving in deciduous habitats and out-competing native species. Their introduction to the UK has had detrimental effects on British forests, and is a constant pain for the average British forester. The greys out-compete the native red squirrels for resources, carry a virus lethal to them and damage trees by stripping bark. This behaviour leads to economic losses estimated at £10 million annually.


Control strategies for grey squirrels in UK forestry have evolved over time. These include shooting, trapping, and formerly Warfarin bait until its EU ban in 2015. Annual estate investment in control efforts varies – more effective management comes to approximately £50 per hectare.

Forestry Journal: Could pine martens be a potential route to controlling the grey squirrel population? Could pine martens be a potential route to controlling the grey squirrel population? (Image: Getty/stock)

While ecological methods like fostering pine martens show potential, they also come with their challenges. Immuno-contraception, although promising, lacks adequate research and funding (as of 2017). It’s important to recognise that bark-stripping behaviour by grey squirrels is not unique – similar actions have been observed globally in various species, including elephants, monkeys, and meerkats.


For the purpose of this analysis, data on grey squirrel damage in the UK was collected from three studies conducted between 1983 and 2014 in Devon, the Chilterns, and Lothian. The study primarily aimed to analyse grey squirrel damage to common tree species and evaluate bark-stripping patterns across distinct regions in the UK. Our analysis used the same species found across all three sites and, using R and SAS, was conducted to investigate variations in bark preference among sites and on a national scale. Adjustments in the analysis were made for birch data due to zero values in one site. Methodological challenges in this analysis include data-processing inaccuracies due to digitising old bar graphs and variations in data-collection timeframes among studies.

Bark-stripping behaviour in grey squirrels across the UK displays variations both nationally and regionally, suggesting a complex interplay of factors including social learning and ecological differences.


Although various hypotheses have been proposed to explain bark-stripping behaviour, such as nutrient deficiencies, aggression in male juveniles, or lack of calcium to name a few, my results, based on similar information about other species, suggest that bark stripping is a behaviour that has been learned. In a more light-hearted tone, grey squirrels in the UK may have a ‘foodie culture’, and we don’t even realise it. Effective management strategies are challenging, and conventional methods have been ineffective. 


There are suggestions to target social learning through interventions during critical learning periods or by provisioning proper nutrition to deter bark-stripping behaviour. However, these approaches require extensive research, resources, and consideration of potential secondary impacts on forest ecology. Innovative and multi-faceted solutions, tailored to specific ecological landscapes and behaviours, would be a pricey but probably necessary way to address bark-stripping behaviour in the future.


According to studies from North America, seasonally inconsistent damage patterns by grey squirrels in their native habitat suggest that it is a learned behaviour. The behaviour is shared either vertically (adult-to-juvenile) or horizontally (peer-to-peer). If these behaviours were not learned, they would be random in terms of tree species and size. 

Bark-stripping behaviour in grey squirrels varies globally, with observations of gnawing on trees like sugar maples and oaks in their native North America. However, introduced populations in places like South Africa exhibit no such behaviour, suggesting regional differences unrelated to genetics.

Further research avenues include replicating studies like Peacock & Jenkins’ (1998) Belding’s ground squirrel experiment to investigate vertical behaviour transmission (adult-to-juvenile teaching) and conducting longitudinal studies to track changes in bark-stripping preferences over time. Experimentally relocating squirrel populations to different habitats could shine a light on the role of social learning versus ecological factors in behaviour. However, legal constraints may limit such experiments in the UK. Analysing geographic differences in behaviour could reveal cultural trends in squirrel populations, challenging biases regarding animal culture and cognition. 

Despite the financial implications, exploring these avenues could deepen understanding and inform more effective management strategies for grey squirrels.

To view Theri Reichlin’s full thesis, visit here.

DISCLAIMER: Our columns are a platform for writers to express their personal opinions. They do not necessarily reflect the views of the writers’ own organisations or Forestry Journal.