Flygekorre – Liito-orava – Pteromys volans has suffered a catastrophic drop in population in Europe since 1950 (Törmälä & Vuorinen 1982, Hyvärinen et al. 2019).
Loss of forests and of older trees with the nest holes that Pteromys volans needs are believed to be the main cause of their decline.(Suzuki et al. 2013, Timm & Kiristaja 2002). Minimally invasive chainsaw techniques developed by foresters are technically and athletically demanding but quickly and efficiently create nest holes with precise entrance and internal dimensions. These have delivered exceptionally strong results in 2025 with endangered Eliomys quercinus and Muscardinus avellanarius in Belgium and Switzerland. Together with retention of food plants it is hoped that they can make a significant difference to the future of Pteromys volans.
Svenska versionen.
Nest Holes
Carved nest holes for Pteromys volans aim to have an entrance 43 mm wide or narrower – 30 mm (Suzuki et al. 2013) or 25mm (Niclas Fritze’n pers. comm. 2026) by approximately 50 to 70 mm high, a depth of up to 300 mm and a height of up to 5 meters above the ground. The experience of local foresters guides the exact height and orientation chosen for entrances and the depth to which nest holes are carved. Foresters often prefer entrances to face away from the direction from which storms usually come. This provides the safety from predators that Pteromys volans need to raise young in the summer and safety from storms and the intense cold of the winter.
Chainsaw carving can be done in dead trees or in living trees of low ecological and economic value. The nest holes are carved entirely through the narrow entrance to ensure minimum damage to the living parts of the tree. With skill and practice, chainsaw techniques allow entrance widths to be carved to an accuracy of +2/-0 mm.
All living trees carved in the Belgian and Swiss field studies have remained healthy. Even in tree species with a high tendency for epicormic growth, the carving does not appear to cause substantial endocrine disruption, and adventitious growth is not observed in the year following carving. Carving entirely through a single narrow entrance allows loss of hydraulic conductivity (Ψ) in living trees to be kept well below the 50 to 88 % (Ψ50 to Ψ88) limit suggested by Hirons & Thomas (2017).
Field study sites have been exposed to storms which have felled other trees however no living or dead tree has failed where the nest hole was carved. Impact on the biomechanical stability of trees is minimised – keeping loss of sapwood well under the 30% limit and loss of heartwood under the 50% limit established by Mattheck et al. (1993).
Naturally occurring nest holes tend to extend in and downwards however chainsaw carved nest holes extend in and upwards – avoiding the risk of water collecting in the hole during heavy rain.
Adoption of carved nest holes has been quick. In the field study organised by Will Robertson and Goedele Verbeylen, endangered Eliomys quercinus and Muscardinus avellanarius moved in to carved nest holes within 12 months of carving – the first time in the world that this has been achieved.
Carving techniques and sustainable forestry practices are developed by foresters to be practical, fast and highly effective. Leadership by foresters has lead to techniques which are practical and efficient to implement at large scale and deliver good results with endangered species. This overcomes the practical and social problems and limited conservation effectiveness of older approaches developed by teams who lacked practical experience in forestry (Wistbacka 2023, Jokinen 2019).
During carving, the angle of the chainsaw is adjusted to achieve wide internal dimensions through a single narrow 20mm entrance incision for Eliomys quercinus. For Pteromys volans a 43 mm entrance width is aimed for.
With sufficiently careful technique, a normal bar can be used however a carving bar with tip geometry designed to minimise kickback risk and a pico chain reduce kickback risk.
Decontamination of equipment using a 70 to 80% by volume ethanol in water solution sprayed onto the chain and bar before replacing the sheath is fast and practical and minimises the probability of disease transmission between living trees.
Depending on the situation, climbing can be by ropes, by ladder or by careful use of climbing spikes. Where climbing spikes are used they are also decontaminated using 70 to 80% by volume ethanol in water.
Carved nest holes show much better thermal stability than nest boxes and are much less visible to both ground-based and aerial predators. ()
Over 3 summers, tree roosting bats (family Vespertilionidae) in rehabilitation at Bat Rehabilitation Ireland and Stiftung Wildstation Landshut in Switzerland showed a strong preference for chainsaw carved nest holes over bat boxes and other roosting opportunities.
Where a tree is already hollow in the centre, an entrance hole can simply be carved to allow access to the hollow.
The Ural owl (Strix uralensis), eagle owl (Bubo bubo), tawny owl (Strix aluco) and goshawk (Accipiter gentilis) are thought to be the primary aerial predators of Pteromys volans (Selonen & Mäkeläinen, 2017, Hanski et al., 2000b; Selonen et al., 2010a).
The forester’s judgment is important both in minimising the visibility of carved nest holes to these aerial predators and in positioning nest holes in areas where obstacles like branches, etc. make it difficult for these predators to fly and hunt.
It is hoped that Pteromys volans will be able to use their sharp teeth and powerful jaw muscles to remove any laminar sections of wood left protruding into the nest hole after carving.
Precise entrance dimensions for Pteromys volans can be based on observations of natural nest holes in Finland – 30 mm wide and about 80-100 mm high (pers. comm. Ralf Wistbacka), 30 mm in Japan (Suzuki et al. 2013) or can be estimated using cranial dimensions (MCPA2). Kim et al. 2021 found that when Pteromys volans were given the choice of 65 mm and 40 mm diameter circular entrances they preferred 40 mm.
Pine marten (Martes martes) is a predator (Partanen, 2011, tr. Robertson). The literature reports differing cranial dimensions with different subspecies of Martes martes however a local hunter in Pietarsaari reports around 46 mm for males or less for females (pers. comm. Ralf Wistbacka 2025) so entrance widths under 46 mm are planned.
Close to human habitations, domestic and feral cats (Felis catus) are a common non-native predator (Mäkeläinen, 2016).
Ground-based predators are also thought to search for nest holes primarily by sight – they do not appear to have the same level of climbing ability as Pteromys volans so the judgment of foresters is critical in positioning nest holes to minimise their visibility to domestic cats and pine martens and to positioning them in places which cats and martens will find difficult to climb to.
In Belgian field studies a domestic cat was able to climb on rough bark to a carved nest hole 3 meters above the ground and 30 mm high x 25 mm wide carved for hazel dormouse (Muscardinus avellanarius) and attempted to push a paw in to the nest hole but was unable to gain access – a mother hazel dormouse successfully moved her family of 4 young into this nest hole.
Pteromys volans, Eliomys quercinus, Muscardinus avellanarius and Sciurus vulgaris are all thought to move between multiple nest holes to reduce the risk of predation so both quantity and quality of nest holes seems to be important. Moving between nest holes may help to control the build-up of ectoparasites in nests however the evidence on this is unclear.
The experience and local knowledge of foresters is important in positioning nest holes close to the seasonally varying range of food plants that Pteromys volans relies on – particularly Acer, Alnus, Betula, Malus and Pinus and Populus that females appear to rely on while nursing young (Carlson, 2021).
Living trees typically mount a healing response in the living tissue around nest hole entrances while animals – particularly birds – attempt to enlarge entrances to their requirements. The long-term entrance dimensions achieved in nest holes appear to result from a balance between trees and animals. The experience of foresters is important in studying how carved nest holes develop in the long term.
Typical Dimensions of Natural Nest Holes
Ingång (a x b): 49 x 45 mm
Framvägg (c): 82 mm
Inre diameter: 122 mm
Bohålets höjd (g,b,e): 300 mm
Höjd över marken: ca 5m
DBH-träd: 400 mm Populus tremula
H = 200-250 mm
Page prepared by Will Robertson and reviewed by Ralf Wistbacka and Goedele Verbeylen.
References to research papers, etc. can be found here: References
“As climbing arborists, we exist at a unique confluence of art, science and athletics. Our profession has undergone a transformation from a dogmatic to a scientific era similar to that in medicine in the second half of the 20th century. As the challenges of biodiversity loss intensify, we find ourselves at a second turning point: we change from being consumers of research to leaders of the fundamental research and of the practical advances needed to ensure that trees remain homes to our most beautiful and endangered creatures.”
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From the tree crowns of an Irish forest I’d like to welcome you. At time of writing in July 2024 we’re focused on field studies, rehabilitation studies, bat counts and trying to work out what best to do after a devastating series of wildfires affecting Myomimus roachi in autumn 2023 and summer 2024 so this is just a basic site and we’ll be adding a lot more detail on species, people and techniques in English, German, French and Italian in autumn 2024. 🙂
Will 🌳🌲❤
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