Through a pilot project, Natuurpunt wants to find out if carved nest holes in trees can offer a rapid alternative to natural tree cavities, which develop over many decades and are in short supply. The target species are our endangered garden dormice (Eliomys quercinus) and hazel dormice (Muscardinus avellanarius). Cavities were carved in five areas in Belgium (Landen, Huldenberg, Borgloon, Voeren and Viroinval) with the help of a Scottish arborist just as a woodpecker would. In Switzerland, high-speed, high-precision minimally invasive chainsaw carving techniques have been developed for foresters and arborists. Soon all sorts of forest animals arrived to inspect them.
In nature, tree cavities form in a variety of ways, often with the help of birds (with woodpeckers as an obvious example) and fungi, through decay where a branch broke off, in cracks and fissures, and under loose bark. Such structures are important for biodiversity and are especially prevalent in older trees, whose preservation is thus much needed. Unfortunately, in our overly cleaned-up world, trees are also often “cleaned up”. Thus, they are not given the chance to grow old and hollow. Or if they are, they are cut down or stripped of their hollow parts, often because of supposed safety hazards. By lending a hand, we can speed up the process and allow suitable structures to emerge in younger trees as well, in what is called “veteranization”.
A well-fed garden dormouse explores a carved tree cavity.
Additional nesting opportunities through carved tree cavities.
Carving tree cavities, in living trees or sufficiently stable dead parts of trees, is something that has begun to receive more and more attention in recent years. For some species, creating additional nesting opportunities can be done quite easily. It is then enough to drill a straight tunnel or use a chainsaw to cut a slit in the trunk, or sheltered from the weather on the underside of a branch. For species that require a small entrance with a large cavity behind it, it is technically more difficult. The usual way is to use a chainsaw to cut a block out of a tree trunk, then replace the outer piece or attach a panel with an entrance hole in it. The disadvantage is that this is quite invasive, and the panel – especially if it is thin – can be pushed in or out by the growth of wound tissue from the tree and even fall out completely. Moreover, the panel consists of dead wood, which decays faster.
Dormice in, predators out.
Less invasive methods currently being developed create tree cavities through a small entrance, just enough to let the targeted species in and keep larger predators out. A special tool for this is already available in Australia, which works through an entrance of at least 5 cm. Will Robertson, a Scottish professional arborist who is also active in Europe, volunteers to work on tools and techniques to keep the entrances even smaller, which better suits our dormouse species (family Gliridae). He came to Belgium in August, to set up a field study together. We went into the field with all kinds of tools, such as a ladder, climbing gear, a cordless drill, power packs to recharge the drill in the field, a compact cordless leaf blower to blow the wood chips out of the cavities, extra long butterfly drills, and a homemade router to mill away in the cavity the remaining partitions between the boreholes.
Refining tree cavity carving technique.
Over 6 days, Will drilled 11 cavities in 5 different areas where hazel dormice and/or garden dormice occur (Landen, Huldenberg, Borgloon, Voeren and Viroinval). The cavities are roughly 150 mm in diameter in all directions, were notched at about 3-4 m height (not higher – to allow monitoring) in a lot of different tree species and wind directions, some with elongated and some with round entrances, with sometimes the cavity below the entrance and sometimes above it. Keeping the entrances small enough (for hazel dormouse 22 mm round or 16 x 40 mm oblong, for garden dormouse 30 to 45 mm round or 20 x 50 mm oblong), worked out better and better with increasing experience. Mostly dead heartwood was removed, and the sap flow is interrupted only by the small entrance. This sap flow is important for hibernators as well as for the tree: the moisture in living trees acts as a buffer, so the climate in the cavities is much more stable than in dead trees or nest boxes and the humidity remains sufficiently high. Carving took 2.5 to 3 hours per cavity, but since then the technique has improved greatly, making it much faster and more accurate.
Nest hole carving by Will Robertson
In addition, Will is also fine-tuning much faster techniques, using high-speed drilling tools with replaceable carbide tips powered by an angle grinder. Or by working with arborist and forestry chainsaws, which allows narrow cuts up to 45 cm deep cut entirely through an entrance as wide as the saw blade (e.g. angled upward for tree dwelling bats (family vespertilionidae)). If desired, these can widen towards the back of the cavity. Again, the shape and size of entrance and inner cavity can be adapted to other species, such as squirrels and dormouse species. However, the latter two techniques are much more dangerous (due to risk of kickback), and require skill and muscle power to perform safely.
Wildlife cameras were installed at most of our 11 cavities, so we can monitor whether and how quickly the dormice start using them, whether other species move in, whether fewer predators come by (the cavities are located higher up and are much less conspicuous than the dormouse nesting boxes we usually use), whether the cavities below the entrance fill up easily with water, whether the wounds heal quickly and the entrances grow back closed quickly, and so on. Questions enough. This first little survey will already provide some more insight into those.
Nuthatch gets in, but almost doesn’t get out.
Immediate results
The tree cavities were immediately visited by a whole host of forest birds (nuthatch (Sitta europaea), short-toed treecreeper (Certhia brachydactyla), marsh tit (Poecile palustris), green woodpecker (Picus viridis), great spotted woodpecker (Dendrocopos major), great tit (Parus major), blue tit (Cyanistes caeruleus), robin (Erithacus rubecula) and wren (Troglodytes troglodytes)). They frequently chased each other away, sometimes making great efforts to get inside, indicating a shortage of suitable tree cavities. At several cavities, woodpeckers started to enlarge the entrance. Red squirrels (Sciurus vulgaris) also sometimes came to look and gnaw at the entrance, and once a pigmy shrew (Sorex minutus) even crawled in.
Just 2 weeks after carving, a first garden dormouse crawled into one of the cavities, which has since been visited 16 times by garden dormice of various ages. In another area, a garden dormouse also already showed interest in a cavity. Now it is waiting until after hibernation to see if the dormice find the cavities suitable to live in. If not, there are already more than enough other candidates. Find many more videos here.
Do it yourself?
If you want to get started on this yourself, get guidance from an arborist or other expert who knows where and where not to drill, so that the stability and survival of the tree is not compromised. Even on capped or dead trees, one or more cavities can easily be carved at the top, and you need not worry as much about the condition of the remaining trunk section. When carving cavities, try to follow up thoroughly on their evolution and use so that we can learn about them.
Text: Goedele Verbeylen (Natuurpunt, Mammal Working Group) and Will Robertson (New Homes for Old Friends)
With help from local volunteers in selecting the sites and installing and monitoring the wildlife cameras: Jules Robijns (Natuurpunt Landen), Frederik Fluyt, Astrid Cervantes, Thomas Vandenberghe and Stijn Verstraeten (Natuurpunt Druivenstreek), Pieter Moysons (Regionaal Landschap Dijleland), Jos Reekmans, Joachim Volont and Stefan Carolus (Natuurpunt Borgloon), Ivo Vanseuningen (Natuurpunt Mammal Working Group), Rian Pulles (Stichting De Slaapmuis), Davy Noelmans (VLM)
With thanks to Forst Seeland and Stiftung Wildstation Landshut in Switzerland and Susan Kerwin at Bat Rehabilitation Ireland.
Translated from https://www.natuurpunt.be/nieuws/boomholtes-kerven-voor-bedreigde-slaapmuizen
German Translation / Deutsche Übersetzung: https://new-homes-for-old-friends.cairnwater.com/de/geschnitzte-baumhohlen-fur-bedrohte-schlafmause/
French Translation / Traduction en français: https://new-homes-for-old-friends.cairnwater.com/fr/cavites-darbres-sculptees-pour-les-muscardins-muscardinus-avellanarius-et-les-lerots-eliomys-quercinus-menaces/