England Released 1,124 Dormice Into English Forests — What Happened Next Shocked Experts

Added: 2026-05-28

[00:00:00]On the morning of June 15th, 2021, a car left London heading north. In the back, in small ventilated containers, were 30 hazel doorm. They weighed about as much as 2 lb coins each. They had golden fur, large dark eyes, and no idea where they were going. By the time the car arrived in Lancaster, about 5 hours later, those 30 animals would make history. The last of them to be released that day was the 1,000th door mouse reintroduced to British woodland since the program began in 1993. That number took 28 years to reach. It happened quietly in a woodland in a protected landscape called the Arnside and Silverdale area of outstanding natural beauty near Morham Bay in front of a small group of conservationists, volunteers, and scientists. No television cameras, no national news coverage. But what those animals had already done in the woodlands where they had been released over the previous 28 years was something that nobody had fully predicted when the program began. In Nottinghamshire, communities had organized themselves around doormies. In Lincolnshshire, a population released years earlier had grown into something that scientists described as healthy and sustainable. In Lancashshire, surveys carried out just two years after a separate release found hundreds of sightings across a two-year monitoring period. The experts involved in this program had hoped the dormice would survive. What they had not counted on was what would change around the dormiss in the woodlands, in the hedros, and in the communities living next to them. This is that story. If this kind of conservation story interests you, subscribe to the channel. We cover restoration ecology and the science behind it, and we try to get the details right. Most people in England have never seen a hazel door mouse. That is not because they are hard to spot in the way that rare birds are hard to spot where you need to travel to the right habitat and get lucky with timing. Dormice are hard to see because they are nocturnal because they spend almost their entire lives in the tree canopy rather than on the ground and because for 6 months of the year from October through to April they are underground in hibernation. A door mouse that is awake and active is still remarkably easy to miss. It weighs between 15 and 20 g, small enough to sit in the palm of a hand with room to spare. Before hibernation, it will have fattened itself to around 35 to 40 g, almost double its summer weight because it has to survive 6 months underground without eating anything. Those reserves have to last until April when hazelnuts and blackberries and honeysuckle flowers appear again and the door mouse can start refueling. The hazel door mouse is the only small British mammal with a fully furry tail. Every other small rodent in Britain has a naked or lightlyhaired tail. The door mouse tail is covered in the same soft golden brown fur as the rest of its body which gives it an unusual appearance almost as if it is wearing a costume. John James Ottabbon's contemporary Gilbert White writing in the 18th century described seeing doormit mice regularly in the hedros of Hampshire. Victorian children kept them as pets which is where the door mouse in Alice and Wonderland comes from curled up asleep in the teapot.

[00:03:00]That detail matters. A door mouse was a normal, expected part of English countryside life 150 years ago. Children kept them. Farmers found their woven nests tucked into hedgerros. They were not rare or special. They were simply there in the background of the rural landscape like robins or hedgehogs.

[00:03:17]Today, the hazel door mouse has disappeared from 17 English counties.

[00:03:21]Its population has fallen by more than 50% since 2000. By some estimates, using more recent monitoring data, the decline is closer to 70%. The species that Victorian children picked up from hedgerros and dropped into flower pots to watch hibernate is now being pushed toward the IUCN's endangered category by the very organization running the program to save it. What happened is a story about three things that all went wrong at the same time. And understanding what went wrong is the only way to understand why getting them back is as complicated as it is. The first thing that went wrong is the simplest to explain. England lost most of its ancient woodland. Ancient woodland is woodland that has been continuously wooded since at least 1600, meaning it has never been cleared and replanted. It has old trees, complex soil, and a shrub layer that developed over centuries. It is a completely different ecosystem from a plantation or a recently planted wood. Ancient woodland takes hundreds of years to develop and cannot be recreated in any meaningful time frame once it is cleared. Since the 1930s, England has lost more than half of its ancient woodland. Some was cleared for agriculture, particularly during and after the Second World War, when food production was the national priority, and the government encouraged farmers to bring every possible acre into cultivation. Some was replaced with commercial conifer plantations, which grow fast, produce timber quickly, and support almost no native wildlife. Some was lost to roads, housing, and industrial development. What remained was often fragmented. Instead of continuous stretches of connected woodland, what was left were isolated patches separated by fields, roads, and developments that a door mouse cannot cross. This matters enormously because a door mouse seldom travels more than 70 m from its nest. That is not a typo. 70 m, roughly the length of a standard swimming pool. A door mouse cut off from other door mice by a field, a road, or a cleared stretch of land is genetically isolated. It cannot find new mates. It cannot recolonize neighboring woodland if the local population crashes. Over generations, isolated dormice become fewer and weaker until eventually there are none. So, the first problem was simple habitat loss. The woodland shrank and fragmented and the dorm mice shrank and fragmented with it. The second thing that went wrong is less obvious and this is the part that most conservation coverage of dorm mice skips entirely. It is a story about a farming practice that almost nobody talks about anymore because the practice itself has almost entirely disappeared. The practice is called caposing. Copasing is an ancient woodland management technique. You cut a tree down to a stump close to the ground. It sounds destructive, but most broadleaf trees when cut this way do not die. They respond by sending up a mass of new shoots from the base. Those shoots grow fast, producing multiple slender stems from a single root system.

[00:06:11]After 7 to 15 years, depending on the species and what the wood is being used for, you cut those stems again and the cycle restarts. For thousands of years, copasing was how English woodland was managed. The cutwood was used for fuel, for charcoal, for fencing, for building materials. Every farm and village had access to copest woodland that produced a steady supply of usable wood on a rotating cycle. The woodland did not disappear. It was used over and over and it stayed. What cpposing also did without anyone particularly planning for it was create exactly the habitat structure that dormmites need. When you cut a section of copest woodland, light floods into the forest floor. Shrubs that were suppressed under the closed canopy suddenly grow fast, filling the open space with blackberries, honeysuckle, hazel, and bramble. Within a few years, that dense, tangled, low growing layer between the ground and the tree canopy, the layer called the understory, is where dormice live. It is where they nest. It is where they find food. It is where they travel through the trees without ever having to touch the ground. A copist woodland with its patchwork of recently cut areas in different stages of regrowth alongside older mature trees creates what ecologists call a mosaic habitat.

[00:07:23]Different stages of growth existing side by side. Each one providing something different. Food, shelter, movement routes, nesting sites. After the Second World War, cheap oil and coal replaced the need for copus wood as fuel. The market for small diameter cppus products collapsed. One by one, landowners simply stopped copassing. The woodland grew on and the tree canopy slowly closed. Shade deepened. The shrubby understory that Dormise depended on faded away. The woodland was still there, still counted as woodland on maps. Still looked like forest from the road, but inside it was becoming inhospitable to dormise. No light, no blackberries, no honeysuckle climbing through the shrubs, no dense tangled branches for a small mammal to travel through without touching the ground. The woodland was present, but it was broken. Ian White, the door mouse and training officer at People's Trust for Endangered Species, described it this way. The loss of habitat quality was the real concern, not just the loss of habitat itself. Woodlands that still exist that appear on maps that people walk through and call forests are in many cases simply no longer suitable for dorm mice because the understory has gone. That is a much harder problem to solve than just planting more trees. The third thing that went wrong happened in the hedgeros and it happened fast.

[00:08:42]Hedros are the networks of dense shrubs and trees that run between fields across the English countryside. They are not just fences. They are ancient features of the landscape, some of them hundreds of years old, and they serve as wildlife corridors connecting isolated patches of woodland. For a door mouse, a good hedge is how you get from one woodland to the next without crossing open ground.

[00:09:03]Between the end of the Second World War and the present day, England and Wales lost approximately 400,000 km of hedro.

[00:09:10]That is a number large enough to be difficult to visualize. Imagine a line of hedro that could circle the Earth 10 times and then some. Gone. removed to create larger fields for modern machinery allowed to deteriorate through lack of management cleared for development. The hedgeross that remained were often poorly managed. A healthy door mouse hedro has height, width, and species diversity. It has mature trees standing out of it at intervals. It has the same dense shrubby structure that copist woodland provides. A thin, frequently cut, single species hedge is almost worthless to a door mouse. It provides no shelter, no food, and no safe route of travel. So by the end of the 20th century, dormise in England faced all three problems simultaneously.

[00:09:55]Less woodland, worse woodland, and a broken network of hedge that could no longer connect the woodland patches that remained. In those conditions, dorm mice could not recover on their own. Not in 17 counties where they had already disappeared. Not even in the counties where they still existed, because those populations were falling steadily yearbyear. A door mouse that cannot travel more than 70 meters from its nest in a landscape that has been cut into thousands of disconnected fragments does not have the ability to find its way back to places it has been lost from. It simply cannot. The people who understood this decided in 1993 that if dorm mice were going to survive in England, someone was going to have to carry them.

[00:10:34]The People's Trust for Endangered Species is a British wildlife charity that has been running the National Door Mouse Reintroduction Program since it began. The partners in that program include the Zoological Society of London, Natural England, Paintton Zoo in Devon, the University of Cumbria, and a network of local wildlife trusts, landowners, and volunteers across England. The basic idea is straightforward. Breed dormise in captivity. Release them into woodlands where dormise used to exist, but no longer do. Monitor them, protect them, manage the woodland around them, and hope that the population takes hold. In practice, every part of that process is more complicated than it sounds. The captive breeding happens at ZSL, Paintton Zoo, Wildwood Trust in Kent, and with a network of licensed private breeders. Dormise produce one litter per year in the wild, sometimes two, with an average of four to five young. They are not easy animals to breed in volume. A cold, wet spring of the kind that England experiences regularly can reduce breeding success across the country in a single season and leave the reintroduction program short of animals.

[00:11:39]This actually happened in 2024 when poor spring weather cut the number of door mice available for release and forced the program to scale back its plans.

[00:11:48]Before any door mouse is released, it spends around 10 weeks in quarantine at ZSL. During that time, ZSL wildlife veterinarian Dr. Tammy Shadbolt and her team carry out full health examinations, blood tests, stethoscope checks, microchips inserted under the skin between the shoulder blades so that each individual can be identified in monitoring surveys after release. Every animal that goes into the wild has been cleared of disease and confirmed healthy. This matters because releasing a sick animal into a clean woodland or an animal carrying a pathogen that wild dormise have not encountered could damage the very population you are trying to build. The woodland that receives the dormise has to meet specific criteria. It needs to have the right structure including shrubby understory and connecting hedgerros. It needs a long-term management plan committing the owner to maintaining the habitat. And the management plan has to include cpposing or some equivalent practice that keeps the understory open and diverse. Releasing door mice into a dark closed canopy woodland with no understory is not conservation. It is just moving animals into unsuitable habitat. This is the part of the program that most news coverage does not mention. Before the dormise arrive, the woodland has to change. The release process itself begins in early summer, usually June. Between 34 and 40 dormise are transported to the site and placed in breeding pairs or trios inside large soft release cages. These cages are secured to trees in the woodland up off the ground where the dormice feel safe.

[00:13:16]Inside the cages, the animals get used to the sounds and smells of their new home. Supplementary food is provided for the first 10 days. Dormise are small, nervous animals. You do not simply open a box and pour them onto the ground.

[00:13:30]After 10 days, the cage doors are opened. The doormice leave on their own terms when they are ready. Typically, within a day or two, all of them are gone, absorbed into the woodland, climbing through the shrub layer, beginning to explore the territory around them. Hundreds of nest boxes are installed at each release site before the dorm mice arrive. These are small wooden boxes with a hole in the front, similar to bird boxes, but positioned in the shrub layer rather than high on tree trunks. Dormis will use them to sleep in during the day and sometimes to raise young. Monthly monitoring visits by volunteers check each box, recording any occupants, weighing them, noting breeding condition. Those records feed into the National Door Mouse Monitoring Program, which has been running since 1990 and is now the longest running small terrestrial mammal monitoring program in the world. More than 120,000 individual records have been collected by hundreds of volunteers across England and Wales since that program began.

[00:14:23]ZSL's wildlife health team continues monitoring after release. If a door mouse dies, a full postmortem examination is carried out to understand why. Each death provides information that makes the next release better.

[00:14:36]Conservation at this scale is not just a single act of releasing animals into a wood and walking away. It is a continuous loop of releasing, monitoring, learning, and adjusting.

[00:14:46]Georgina Gerard, a wildlife health technician at ZSL who has supervised multiple door mouse releases, described what the volunteer network does as essential. Year after year, she said, a network of local volunteers continue to monitor these little guys at each site where they have been released and find so many flourishing in their new environment. That word flourishing is the one that matters. The first reintroduction took place in Cambridge Shshire in 1993. By the time the program reached its 1,000th door mouse in June 2021, it had released animals into 25 different woodlands across 13 English counties. The Natural England report assessing the program's results found that all sites were successful in the short term. All of them. Every woodland that received dormise saw the animals survive their first two winters and begin breeding in their third year. That is not a given for any reintroduction program. The success rate in the first phase exceeded what the organizers had hoped for. Twothirds of sites remained successful in the medium-term, meaning stable or growing populations confirmed 5 to 10 years after release. Chambers Farmwood in Lincolnshshire is one of the sites that made that 2/3. A population released there years earlier was later found through survey work to have developed into what researchers described as a healthy and sustainable population. The dormise had bred, survived winters, expanded through the woodland, and were using the habitat in the way a naturally established population would. Ian White of PTS said of results like that, one dormice became extinct over half of their former range in Britain over the course of a century that they have been restored in part in some counties as a testament to a unified conservation effort that will require continuing commitment. Here is where the story goes somewhere that nobody planned for. In Nottinghamshire, three door mouse reintroductions were carried out in woodland sites within a fivemile radius of each other. The idea was to eventually create a network of connected populations that could exchange individuals through the hedro corridors between them, building what ecologists call a metapopul, a group of separate populations linked enough to help each other survive. What happened around those Nodding Hampshire sites was not something that had been in any project plan. Local people became interested not just in the dorm mice themselves, but in the woodlands where the dorm mice lived and what those woodlands needed. A local door mouse group formed, bringing together volunteers who wanted to help monitor the animals. That group began working directly with the Nodding Hampshire Wildlife Trust to manage the woodlands more appropriately and more significantly to restore hedros in the wider landscape around the release sites. Hedgero restoration. The very thing that fragmented door mouse habitat needs most. Not because a government agency required it. Not because a conservation plan had specified it.

[00:17:31]Because a group of local people who had become invested in a small goldencoated animal started asking what the landscape around it needed and then went out and started fixing it. This is the part that researchers who have spent decades working in wildlife conservation will tell you is genuinely surprising. The dormise changed how people in Nottingham Shire related to their local woodland.

[00:17:50]And the people changed the woodland in ways that will outlast any individual reintroduction program. The same pattern emerged in Warwickshire, in North Yorkshire's Winsleyale Valley, and in Lancaster. Each reintroduction became not just an ecological event, but a community event, a focal point around which local people organized themselves around the health of their local landscape. Katherine Walsh, senior environmental specialist at Natural England, described the effect in Bedfordshire, where another release took place. The hazel door mouse is one of Britain's most iconic native species.

[00:18:21]She said, "The success of established door mouse populations shows that by working together, we can bring our native species back from the brink.

[00:18:28]Releases accelerate the recovery of this species by creating genetically diverse populations that in turn support a wider web of biodiversity." That last phrase is worth holding on to. A wider web of biodiversity. Here is what that means in practice. The hazel door mouse is what ecologists call an indicator species.

[00:18:47]That means its presence tells you something about the health of the whole ecosystem around it. Dormis do not survive in poor quality woodland. They need the dense understory, the diverse shrub layer, the connected hedge. They need enough honeysuckle and hazel and blackberry to feed through summer and fatten themselves for winter. If dormice are thriving in a woodland, it means that woodland has at minimum the structural diversity and food supply that supports one of England's most demanding small mammals. And those same conditions, the shrubby understory, the diverse food plants, the connected canopy support dozens of other species at the same time. Woodland birds that need dense cover, invertebrates that depend on specific plants, bats that hunt insects through the shrub layer.

[00:19:29]The drywitch standards coverage of the door mouse monitoring program put it plainly. Where there are door mice, there is usually a wider animal and plant diversity. This is why the reintroductions do more than restore doorm. When a release site requires that copassing be restarted and maintained, that management change benefits the entire woodland community. When communities start restoring hedros to help dormice disperse, those hedros become wildlife corridors for every species that needs to travel between woodlands. The door mouse is not just a beneficiary of better woodland management. It is the reason better woodland management happens in places where it otherwise would not. In Lancaster, the picture became vivid very quickly. 69 dormice were released in 2021 as part of a wider species recovery project called Back on Our Map, which aimed to restore 10 locally extinct species to the Morcane Bay area. Surveys carried out in 2023, just 2 years after the release recorded hundreds of door mouse sightings across the monitoring period. The population was not just surviving. It was multiplying and spreading through the landscape at a rate that surprised the people monitoring it. Now, here is the part of this story that every honest account of Door Mouse recovery has to include.

[00:20:38]Despite everything described above, despite the 1,000 animals released, despite the successful populations in Lincolnshshire and Nottinghamshire and Lancasher, despite the community engagement and the woodland management improvements, the overall national population of hazel dormice in England is still falling. It has fallen by more than 50% since 2000. By more recent estimates, the figure is closer to 70%.

[00:21:01]PTE, the organization leading the reintroduction program, is currently advocating for the door mouse to be upgraded from vulnerable to endangered on the IUCN's list of threatened species. 17 counties in England no longer have any door mice at all. The areas where they still exist are almost entirely south of a line running between Shropshshire and Suffach. The reintroduction program is putting animals back into the landscape at a rate of roughly 40 to 50 per year across a handful of sites. The pressures pulling dormise out of the landscape, the loss of woodland understory, the broken hedro network, the changing climate causing dormise to emerge too early from hibernation when food is still scarce are still operating at a national scale across tens of thousands of square miles. Ian White of PTS says the numbers tell a clear story. The program is working at the sites where it operates. The problem is that it cannot operate everywhere and the underlying drivers of decline have not been reversed. Warm winters which climate change is making more frequent are a particular problem. Dormise hibernate because there is no food available during winter and their metabolism drops to almost nothing to conserve the fat reserves they have built up through summer and autumn. A warm spell in January or February can cause a door mouse to wake from hibernation too early. It is now burning energy it cannot replace because the hazelnuts and blackberries are still months away. Some animals starve before spring. Others emerge weakened and fail to breed successfully. A single warm winter can knock years off a population's recovery.

[00:22:31]This is a problem that no reintroduction program can fix. Releasing animals into a well-managed woodland makes no difference if the climate kills them through premature emergence from hibernation. It is the same challenge facing every small mammal that hibernates in a warming country. So where does this leave the door mouse?

[00:22:48]The honest answer is in a better place than it would be without the program and in a worse place than it needs to be for long-term security. The program has now released more than 1,100 door mice into 26 sites across 13 counties. Several of those populations are established and growing. Local communities have built monitoring networks around reintroduction sites that will continue to function regardless of what any government agency does or does not fund.

[00:23:13]The National Door Mouse Monitoring Program, with its 35 years of volunteer collected data and more than 120,000 records, gives scientists a more detailed picture of doormouse population trends than exists for almost any other small mammal in Britain. The techniques being developed, the softrelease cages and trees, the pre-release quarantine protocols, the monthly nest box monitoring, the woodland management standards required before a site can receive animals are also informing reintroduction programs for other species in Britain and elsewhere. Ian White's work at PTEES has become a reference point for how to run a small mammal reintroduction program with limited resources and a large volunteer workforce. What still needs to happen is bigger than any single organization can deliver. The hedro network across England needs to be restored and reconnected. Caposing needs to come back to far more woodland than currently receives it. The planning system needs to treat ancient woodland and door mouse habitat as seriously as it treats buildings and roads. Farmers who manage hedros need support to manage them in ways that benefit wildlife, not just ways that are cheapest and fastest. None of that is simple. All of it is possible. The door mouse has been disappearing from England for over a century so slowly that most people never noticed. The Notting Hampshire volunteers who formed a local group around a woodland reintroduction site noticed. The landowners in Lancaster who agreed to manage their woodland for a small nocturnal animal, most of them had never seen noticed. The ZSL veterinarians who carry out health checks on animals that weigh as much as 2 lb coins, treating each one is worth the time and care, noticed. 30 door mice arrived in Lancaster in a car from London on June 15, 2021. The 10,000th door mouse was released into a woodland near Morham Bay and disappeared into the shrubs. 2 years later, hundreds of sightings were recorded in that same landscape. The dorm mice had been living there the whole time. It just took the right woodland, the right management, and people paying close enough attention to find them. What do you think about programs like this one? the ones that work slowly, that depend on volunteers and patient science, and that may take generations to fully succeed. Leave a comment below. I read everyone. And if you want more stories about how ecosystems recover and what it actually takes to make that happen, subscribe to the channel. This is what we cover.