Squirrels occupy an enormous range of habitats across every continent except Antarctica and Australia, and the shelters they build are just as varied. Tree squirrels weave leafy nests called dreys high in the canopy or commandeer natural tree cavities. Ground squirrels dig tunnel systems that can stretch several meters underground. Flying squirrels crowd into hollow trees in winter, huddling together for warmth. Even within a single species, where and how a squirrel lives shifts with the season, the climate, and what real estate happens to be available. The diversity of squirrel housing reflects a family of rodents that has colonized nearly every terrestrial environment on Earth.
Tree Squirrel Dreys and How They Are Built
The most familiar squirrel home is the drey, the globe-shaped nest of twigs and leaves wedged in a tree fork. If you have ever looked up in winter and noticed a messy ball of brown leaves clinging to bare branches, you were almost certainly looking at a drey. These nests are built primarily by tree squirrels such as the eastern gray squirrel and the Eurasian red squirrel, though some flying squirrels build them too.
A hidden-camera study of a free-ranging red squirrel documented the entire construction process. The squirrel completed the nest over three days, spending a total of about 3.6 hours of active work. The project began with a phase dominated by hauling material to the site, then gradually shifted to weaving the outer twig shell and processing softer materials for the inner lining. Construction happened after several dry days and at temperatures well above freezing, suggesting squirrels may time their building around favorable weather.1Hystrix, the Italian Journal of Mammology. The process of drey construction in red squirrels – nestbox observations based on a hidden camera
The finished product has a layered architecture. The outer shell of interlocking twigs provides structural support and some weather resistance. Inside that sits a core of leaves, moss, and grass. The innermost lining is the softest layer, often made of shredded bark, fur, or other fine fibers. This layering creates a microclimate inside the drey that can be considerably warmer than the outside air, which matters a great deal on winter nights.
Tree Cavities and Why Squirrels Prefer Them
When given a choice, many squirrel species favor natural tree cavities over open-air dreys. A cavity offers better insulation, more protection from wind and rain, and greater concealment from predators. For female squirrels raising young, cavities provide a more stable thermal environment for vulnerable pups.
Southern flying squirrels nest most often in deciduous tree cavities, turning to dreys less frequently than their close relatives, the northern flying squirrels.2Wildlife Research. Seasonal nest use of sympatric North American flying squirrels A study of both species in central Ontario found that the majority of nest trees were in declining or dead wood. Among southern flying squirrels, about 71% of nest trees were declining and 22% were snags (standing dead trees). Northern flying squirrels used snags and declining trees at similar rates, and nearly half of their repeatedly used nests were in snags.3Oxford Academic (Journal of Mammalogy). Nest-Tree Use by Northern and Southern Flying Squirrels in Central Ontario This preference for aging and dead trees has real consequences for forestry, because timber harvest removes exactly the trees squirrels depend on most.
In recently harvested forest, researchers found signs that nest supply became limited because cavity trees and snags were scarce. Southern flying squirrels partially compensated by moving into abandoned woodpecker holes or by packing more individuals into fewer available cavities.3Oxford Academic (Journal of Mammalogy). Nest-Tree Use by Northern and Southern Flying Squirrels in Central Ontario Hardwood snags and decaying trees appear to be particularly important for breeding females, which need stable, well-insulated nest sites to raise pups successfully.
Ground Squirrel Burrows
Roughly half of all squirrel species spend most of their lives on or under the ground rather than in trees. Ground squirrels, prairie dogs, marmots, and chipmunks all excavate burrow systems that serve as sleeping quarters, nurseries, food caches, and hibernation chambers. The size and complexity of these tunnels vary enormously.
A comparative analysis of ground-dwelling squirrel species found that body size was the strongest predictor of burrow dimensions. Larger species dug longer and deeper tunnels, likely because they need more underground space to move around. Burrow length scaled with body mass in a way that suggested a natural ceiling imposed by ventilation: in a long, narrow tunnel, fresh air has trouble reaching the far end, so there are metabolic limits to how far a squirrel can dig. The study also found that more social species excavated longer burrow systems, probably because multiple residents sharing a tunnel network need more room.4Oxford Academic (Journal of Mammalogy). Factors influencing burrow length and depth of ground-dwelling squirrels
A typical ground squirrel burrow has multiple entrances, which serve as escape routes from predators and help with airflow. Inside, tunnels branch into distinct chambers for nesting, storing food, and defecating. Some species plug certain entrances with soil during hibernation to reduce airflow and stabilize temperature, essentially sealing themselves in for the winter.
Surviving Extreme Cold Underground
Arctic ground squirrels push the limits of what a mammal can endure. They hibernate in underground burrows where soil temperatures plunge well below freezing. Researchers who implanted temperature-sensitive transmitters in free-living Arctic ground squirrels found that abdominal body temperatures during hibernation reached average lows of about −2°C, while soil temperatures at the depth of the hibernation chamber dropped to around −6°C.5PubMed. Freeze avoidance in a mammal: body temperatures below 0 degree C in an Arctic hibernator The squirrels avoid actually freezing solid through a process called supercooling, where body fluids stay liquid below their normal freezing point.
To survive months of this, Arctic ground squirrels prepare their hibernacula carefully. They line the nesting chamber with dried grasses and other insulating materials, creating a pocket that buffers temperature swings. Even so, they periodically arouse from torpor, briefly warming their bodies back to normal temperature before dropping down again. These arousals are metabolically expensive but appear necessary to clear waste products and reset certain physiological processes. The burrow itself is the critical piece of infrastructure that makes this entire survival strategy possible.
Desert Burrows as Climate Refuges
At the other thermal extreme, ground squirrels in arid environments use burrows as cooling stations. Desert antelope ground squirrels forage on scorching surfaces where ground temperatures can exceed what their bodies can safely handle. They solve this by shuttling between the hot surface and cool underground retreats, alternately heating up while foraging and dumping heat in their burrows.6ScienceDirect. Optimal body temperatures with shuttling: desert antelope ground squirrels The burrow acts like a personal air conditioner: just a short distance underground, temperatures are dramatically cooler than the baked surface above.
This shuttling behavior means desert squirrels are rarely far from a burrow entrance. Their entire foraging range is constrained by the need to get underground quickly when body temperature rises too high. The spacing and availability of burrows effectively determine how much of the landscape a desert ground squirrel can use.
Communal Nesting and Huddling for Warmth
Many squirrel species that are otherwise solitary will share nests during cold weather. This behavior, called communal nesting, is driven primarily by the need to conserve body heat. In Abert’s squirrels, the proportion of individuals nesting communally increased as ambient temperatures dropped. When breeding season arrived, communal nesting declined sharply, suggesting the arrangement is about thermoregulation rather than social bonding.7Ethology. Communal Nesting in Asocial Abert’s Squirrels: the Role of Social Thermoregulation and Breeding Strategy
Flying squirrels take communal nesting to an impressive level. Southern flying squirrels form large aggregations inside nest-lined tree cavities during winter. Measurements showed that at 9°C, huddling in groups of three reduced energy expenditure by about 27%, and groups of six saved roughly 36%.8Canadian Journal of Zoology. Winter energy expenditure and the distribution of southern flying squirrels This is a substantial saving for a tiny animal that cannot easily put on enough fat to ride out winter alone. Cold temperatures drive up both aggregation sizes and the need for large-diameter cavity trees that can hold the group.2Wildlife Research. Seasonal nest use of sympatric North American flying squirrels
The implication is that a flying squirrel’s winter survival depends not just on finding a nest site, but on finding one big enough to share. A narrow cavity that fits one squirrel comfortably may be useless for a group of six. Logging practices that remove large, old, cavity-bearing trees can therefore hit flying squirrel populations especially hard in winter, when communal nesting is most critical.
What Squirrels Look for in a Nest Site
Squirrels are not haphazard about where they set up home. Research on Arizona gray squirrels identified several consistent patterns in nest-site selection. These squirrels chose areas with more large trees, more standing dead wood, more downed logs, greater canopy cover, and lower slope. Within those areas, they picked tall trees with crowns that interlocked with neighboring trees and tended to place nests close to the main trunk.9The Journal of Wildlife Management. Importance of scale in nest‐site selection by Arizona gray squirrels The researchers concluded that squirrels were selecting for protection from both predators and weather, as well as proximity to food sources.
Interlocking canopy is worth highlighting because it serves as an aerial highway. A squirrel nesting in a tree whose branches touch many neighbors can move through the canopy without descending to the ground, where it is far more vulnerable to terrestrial predators. Conversely, an isolated tree surrounded by open ground is a poor nest site even if it has a perfect cavity, because the squirrel has to cross exposed terrain every time it leaves.
Thermal considerations also shape nest-site decisions in subtler ways. A study of lactating red squirrels found that when ambient temperatures were warm, mothers nursing large litters chose less insulated nests. This finding suggests squirrels actively manage heat load through nest selection, picking cooler, more open nests when overheating is a bigger risk than staying warm.10PubMed Central. Lactating red squirrels experiencing high heat load occupy less insulated nests Nest choice is not simply about maximizing warmth; it is about finding the thermal sweet spot for current conditions.
Nest Materials and Possible Anti-Parasite Benefits
The materials squirrels choose for their nests are not random either. In a southern Ontario hardwood forest, researchers collected 32 nests built by northern flying squirrels and North American red squirrels. They found that roughly 86% of flying squirrel nests and 78% of red squirrel nests were constructed almost entirely of shredded bark from eastern white cedar. The researchers noted that white cedar has known antiparasitic properties and suggested that lining nests with cedar bark may be a behavioral adaptation to reduce ectoparasite loads in the nest environment.11The Canadian Field-Naturalist. Cavity Nest Materials of Northern Flying Squirrels, Glaucomys sabrinus, and North American Red Squirrels, Tamiasciurus hudsonicus, in a Secondary Hardwood Forest of Southern Ontario
This idea of self-medication through nest construction is not unique to squirrels. Many bird species incorporate aromatic plants with insecticidal or antibacterial properties into their nests. If squirrels are doing something similar with cedar bark, it would mean their choice of building material is shaped by parasite pressure, not just availability or insulating quality. The finding is suggestive rather than conclusive, but it adds an interesting dimension to how we think about squirrel nest-building.
Squirrels in Cities and Buildings
Urban and suburban environments present squirrels with a different set of options. Attics, soffits, wall voids, chimneys, and outdoor sheds all provide shelter that in many ways resembles a tree cavity: enclosed, elevated, dry, and protected from wind. Eastern gray squirrels and fox squirrels are especially well adapted to city life and will readily move into human structures when natural cavities are scarce.
For homeowners, the practical reality is that a squirrel views your attic the same way it views a hollow tree. If there is an entry point, the space is dry, and the temperature is stable, a squirrel will chew its way in, shred insulation for nesting material, and set up a nursery. Signs of attic occupation include scratching or scurrying sounds during early morning and late afternoon (when tree squirrels are most active), gnaw marks around roof edges or utility entry points, and droppings resembling dark, oblong pellets.
Urban parks and tree-lined streets often support surprisingly dense squirrel populations. City trees are widely spaced compared with forest canopy, so dreys and cavity nests in urban areas tend to be in isolated trees along streets or in park settings. Squirrels compensate for fragmented canopy by spending more time on the ground and crossing open areas that a forest squirrel would avoid. Supplemental food from bird feeders, garbage, and deliberate human feeding also offsets the lower natural food availability in many urban habitats.
Competition Between Species for Nest Sites
When two squirrel species share a landscape, competition for nesting habitat can have serious consequences for the less dominant one. The clearest example is the replacement of Eurasian red squirrels by introduced eastern gray squirrels across much of Britain, Ireland, and northern Italy. Research found that in sites where gray squirrels were present, red squirrel females had lower body mass, produced fewer litters per year, and showed reduced recruitment of juveniles into the population. In mixed broadleaf woodlands, juvenile red squirrel residency decreased as gray squirrel density increased.12Journal of Animal Ecology. Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics
The competition is not only about nests directly. Gray squirrels outcompete reds for food in broadleaf and mixed forests, which reduces the body condition of red squirrels and makes them less able to reproduce. But nesting habitat plays a role because both species rely on the same tree cavities and sometimes even dreys. Where gray squirrels dominate, red squirrels are pushed into conifer forests where grays fare less well, effectively restricting where red squirrels can live.
Why Dead Trees Matter More Than You Would Think
A recurring theme across squirrel ecology is the importance of dead and dying trees. Snags and declining hardwoods provide the cavities that tree squirrels and flying squirrels depend on for nesting, especially for raising young and surviving winter. Forest management practices that remove standing deadwood, whether for timber, fire safety, or aesthetic reasons, can strip a landscape of critical squirrel habitat even if plenty of living trees remain.
Both northern and southern flying squirrels in Ontario relied heavily on declining trees and snags for nest sites, and females showed an even stronger association with these structures than males.3Oxford Academic (Journal of Mammalogy). Nest-Tree Use by Northern and Southern Flying Squirrels in Central Ontario In harvested areas with fewer cavity trees, flying squirrels were forced to aggregate into the remaining available sites, which could increase disease transmission and social stress. Conservation guidelines for squirrel habitat increasingly recommend retaining a minimum number of snags and large-diameter cavity trees per hectare during logging operations.
For backyard wildlife enthusiasts, the lesson is straightforward: if a dead or hollow tree on your property is not a safety hazard, leaving it standing provides housing for squirrels, woodpeckers, owls, and a host of other cavity-nesting wildlife. A perfectly manicured landscape with no dead wood is, from a squirrel’s perspective, a housing desert.
Seasonal Shifts in Where Squirrels Live
Many squirrels do not stay in one nest year-round. Tree squirrels often maintain several dreys and cavity nests within their home range and rotate among them. A squirrel may use a leaf drey during warm months, when the open structure provides better ventilation, and switch to a more insulated tree cavity as temperatures drop. Mothers frequently move their young between nests, possibly to stay ahead of parasites that build up in bedding material or to avoid predators that have located a nest.
Flying squirrels shift both their nest type and their social arrangement seasonally. During summer, individuals tend to nest alone or in small groups, often in dreys or small cavities. In winter, they consolidate into large aggregations in the biggest available cavities.2Wildlife Research. Seasonal nest use of sympatric North American flying squirrels The transition is driven by falling temperatures and the energy savings of huddling, as described earlier. This seasonal flexibility means that a habitat needs to provide a range of nest options to support a squirrel population year-round, not just a single type.
Ground squirrels show the most dramatic seasonal pattern. Species that hibernate effectively disappear underground for months, sealing themselves into deep chambers while the world above freezes. When they emerge in spring, they reopen burrow entrances and begin the cycle of foraging, breeding, and fattening up for the next winter. In warm months, these same burrows provide daytime shelter, pup-rearing space, and refuge from aerial predators. The burrow is the one constant in a ground squirrel’s life, and losing access to it, whether through flooding, plowing, or development, can be fatal.