Home, Sweet Home! Shelters and Nests for Bats, Small Rodents and Bees
Bat Houses
Some bat house boxes (like the one shown in the picture above) are made up of a specialized mixture of wood shavings (75%), lime, cement, and other additives that make it possible to create box models in every conceivable shape.
Bats may be among the most misunderstood animals that visit our yards. They are not blind, they do not deliberately fly into people’s hair, and the overwhelming majority have no interest in humans. What they are is remarkable: bats are the only mammals capable of true, powered flight. In the United States, most species feed on insects, and their nightly activity contributes to healthy forests, wetlands, farms, and gardens.
Many North American bats use different shelters at different times of year. During summer, females of some species gather in warm maternity roosts where they give birth and raise their pups. As winter approaches, bats may migrate to milder regions or hibernate in caves, mines, rock crevices, hollow trees, and suitable parts of buildings. A standard backyard bat house is primarily a warm-season roost; except in areas with mild winters, it should not be expected to provide the stable conditions bats need for hibernation.
Bats in summer…
A bat house can provide useful summer shelter, especially where old trees, loose bark, natural cavities, or traditional building roosts have disappeared. It will not suit every bat: tree-roosting species such as hoary bats and eastern red bats usually shelter among foliage or beneath bark rather than inside artificial boxes. Colonial, crevice-roosting species—including big brown bats, little brown bats, and Yuma myotis within their respective ranges—are more likely to investigate a purpose-built bat house.
Begin with a genuine bat-house design rather than trying to convert a small birdhouse. The most useful models are tall, weather-tight maternity houses with at least four chambers, ventilation near the lower portion, and an open entrance along the bottom. The interior should be made from untreated material and have roughened or closely grooved surfaces that bats can grip. Narrow spaces imitate the cracks beneath loose bark: chambers about ¾ inch wide suit smaller species such as little brown bats and Yuma myotis, while big brown bats may use spaces between 1 and 1½ inches wide. A textured landing area beneath the entrance helps arriving bats climb into the roost.
Location matters just as much as construction. Mount the house on a building or sturdy pole, generally 10 to 16 feet above the ground, with an unobstructed flight path in front of and beneath the entrance. Avoid mounting it on a tree, where branches create shade, obstruct access, and give predators an easy approach. Bat Conservation International recommends leaving at least 12 feet of open space below the exit whenever possible.
The correct amount of sunlight depends on climate. In hot parts of the South and Southwest, morning sun, afternoon shade, good ventilation, and a light-colored exterior help reduce overheating. Cooler regions may require greater solar exposure and a somewhat darker finish. Installing two nearby houses with different orientations can give bats a choice of temperatures as weather conditions change. Properties with outdoor cats are poor locations because bats must drop from the house before taking flight and can be vulnerable near the ground.
Do not be discouraged when a new house remains empty. Bats may take several seasons—and occasionally as long as five years—to discover and accept an unfamiliar roost. Protecting mature trees and safely retaining dead or declining trees is generally more valuable than installing artificial shelters, so a bat house should supplement natural habitat rather than replace it.
Small, single-chamber boxes can heat and cool too rapidly. Tall, vented, multi-chamber houses provide a wider range of temperatures and are the better choice for most yards. During very hot weather, watch for bats crowding around the entrance, fanning their wings, flying in daylight, or pups falling from the house—possible signs of heat stress. Temporary shade may be needed, followed by improvements to the house or its location.
You’ll know them by their calls
The United States is home to about 47 bat species, and distinguishing them in flight is rarely easy. Size, silhouette, flight pattern, habitat, and range can offer clues, but darkness and rapid movement make visual identification uncertain. Echolocation provides another way to investigate. Bats emit high-frequency calls and listen for the echoes returning from insects and surrounding objects; most of these sounds are ultrasonic and cannot be heard unaided by people.
A handheld or smartphone-compatible bat detector can convert those calls into audible sound or record them as digital files and spectrograms. The frequency, duration, rhythm, and shape of a call may narrow the possibilities, while stored recordings can be examined later or shared with an experienced observer.
Treat automatic identifications cautiously, however. Several North American species produce calls with overlapping characteristics, and background noise, vegetation, distance, and hunting behavior can alter a recording. Detector software and identification apps are excellent for confirming that bats are active and suggesting likely species, but difficult recordings may require expert review before a species-level identification can be considered reliable.
Ultrasonic detectors convert sounds at frequencies inaudible to the human ear into audible sounds. Different species of bats can be identified using an ultrasonic detector because each species uses a different frequency, and the detector provides this information.
BAT HOUSES
Bottom-entry houses for poles or open structures
Many modern bat houses are open at the bottom. Guano falls directly to the ground, so little routine cleaning is required, although the house should still be checked for damage and cleaned only when it is unoccupied. The open base also lets bats drop into flight without having to crawl through a small entrance hole.
Despite the traditional image of a bat house hanging from a tree, trees are usually not the best mounting sites. Branches create obstacles, foliage produces too much shade, and trunks may give raccoons, snakes, and other predators easier access. In most yards, a sunny wall or a sturdy pole in an open area offers better conditions. The entrance should be at least 10 to 16 feet above the ground, with approximately 12 feet of unobstructed space beneath and in front of it.
The most useful designs contain several narrow chambers rather than one large empty space. Each chamber develops a slightly different temperature, allowing bats to move toward warmer or cooler areas as outdoor conditions change. This is especially valuable during spring and fall, when temperatures fluctuate sharply, and during summer, when females are raising pups.
Interior surfaces must be rough enough for bats to grip. Shallow horizontal grooves, closely spaced wooden baffles, or other durable textured surfaces work well; smooth plastic or polished wood does not. Removable panels may be helpful in research or professionally monitored installations, but homeowners should never open or disturb an occupied bat house. Any maintenance should wait until the bats have left for the season.
For walls and buildings
Wall-mounted nursery houses are especially useful for bat species that already roost in human-made structures. A building provides greater thermal stability than a tree or exposed pole, and a large wooden, brick, or concrete surface retains warmth after sunset. These conditions can make wall-mounted houses attractive to maternity colonies and other groups seeking a secure daytime refuge.
Choose a high, unobstructed section of wall where bats can approach and leave without passing through dense branches, wires, or outdoor furniture. Morning and midday sun are often beneficial, but the amount of solar exposure should match the local climate. Houses in cool northern regions generally need more sun, while those in the hottest parts of the South and Southwest may require protection from intense afternoon heat. Installing two houses with different exposures can provide a broader range of temperatures.
The box should be large enough to retain heat and should contain multiple narrow roosting spaces. Tiny, birdhouse-sized models and oversized single chambers are less likely to provide the stable range of temperatures bats need. The landing area and chamber walls should be deeply grooved or otherwise textured so that even young bats can climb easily. Open-bottom designs prevent most guano from accumulating inside, although the ground below may occasionally need to be cleaned.
Place the house away from bright security lights and keep cats indoors, particularly around dusk and dawn. A nearby pond, stream, wetland, woodland edge, or native garden can improve the surrounding habitat by supplying drinking water and night-flying insects. However, installing a box does not guarantee occupancy. Bats may take several years to discover and accept a new roost, especially when it is not replacing an established site.
Built-in bat roosts
Bat roosting blocks and “bat bricks” can be incorporated directly into the exterior walls of houses, apartment buildings, schools, and bridges. From the outside, they resemble ordinary masonry units with a narrow entrance near the base. Inside, they contain textured cavities where bats can cling and shelter.
Built-in roosts are most successful when included in the original architectural plans. They should be high above the ground, protected from rain, free of artificial lighting, and connected only to the outdoors—not to attics or occupied rooms. Materials inside the cavity should be durable, rough, and free from toxic wood treatments or exposed adhesives.
Before renovating, sealing, or demolishing a structure that may contain bats, arrange for an appropriate wildlife inspection. Exterior gaps must not simply be closed while bats are inside, because adults or flightless pups could become trapped. Exclusion work should follow state wildlife regulations and be planned with a qualified bat specialist, particularly when a maternity colony or protected species may be present.
BATS IN THE CITY? • Common building-roosting bats
The species found around homes and city buildings differ greatly by region. These are several bats commonly associated with human-made structures in parts of the United States:
| Common name | Scientific name |
|---|---|
| Big brown bat | Eptesicus fuscus |
| Little brown bat | Myotis lucifugus |
| Mexican free-tailed bat | Tadarida brasiliensis |
| Evening bat | Nycticeius humeralis |
| Yuma myotis | Myotis yumanensis |
Little brown bats commonly use warm attics and bat houses across northern portions of the continent; Mexican free-tailed bats form conspicuous colonies in buildings and beneath bridges in the South and West; evening bats are most widespread in the Southeast; and Yuma myotis frequently roosts near water in western buildings and bridges.
Shelter Designs for Flying Squirrels and Other Small Mammals
Wooded yards throughout North America are home to small mammals that need secure places to rest, raise young, and escape winter weather. Flying squirrels may use artificial nest boxes where natural tree hollows are scarce. At ground level, shrews and other small wildlife benefit more from sheltered piles of logs, branches, and fallen leaves than from enclosed “houses.”
Southern and northern flying squirrels sometimes occupy boxes originally intended for small cavity-nesting birds. These nocturnal animals are easily overlooked, even when they live close to homes. Southern flying squirrels occur across much of the eastern United States, while northern flying squirrels are associated mainly with northern forests and higher elevations. Both rely heavily on cavities in mature trees, including abandoned woodpecker holes, but they may readily accept a properly designed nest box.
A box used by chickadees or wrens during spring and summer may later provide shelter for a flying squirrel. During winter, several squirrels may share the same cavity, especially in colder parts of their range. They do not remain asleep for the entire season, however. They emerge periodically to forage and may stay inside for several days when storms or very low temperatures make travel difficult.
A suitable flying-squirrel box can be made from untreated, rough-sawn wood. The interior floor should measure approximately 4 to 10 inches square, with a total interior depth of about 7 inches. Make the entrance only 1¼ to 1½ inches across and position it high on the side nearest the tree trunk. Small drainage holes in the floor and ventilation openings near the bottom help keep the interior dry. Mount the box roughly 10 to 20 feet above the ground on a mature tree in a wooded or densely planted area. A few dry leaves may be placed inside, but avoid cloth, string, dryer lint, or other materials that could entangle an animal.
Do not open a box during freezing weather or while it may contain young. Instead, watch quietly from the ground around dusk and look for fresh nesting material or signs of activity near the entrance. Clean a box only when you are certain it is vacant. Wear gloves, avoid stirring up dust, and never sweep or vacuum dried droppings or nesting material; potentially contaminated debris should first be thoroughly dampened with disinfectant.
At ground level, short-tailed shrews and related species fill an ecological role somewhat comparable to that of hedgehogs in European gardens. These tiny, secretive insectivores move beneath vegetation and leaf litter in search of beetles, caterpillars, spiders, slugs, snails, and earthworms. They remain active throughout the year and may occasionally be heard rustling through leaves after dark.
Shrews rarely benefit from a miniature enclosed house. A better shelter is a low pile of natural woody material. Begin with several untreated logs or thick branches laid loosely on the ground. Add a second layer at right angles so that small passageways remain between them, then cover the structure with thinner branches. Allow fallen leaves to gather around the edges without completely blocking the openings.
Place this shelter in a quiet, well-drained corner beneath native shrubs or along the edge of a wooded area. Shade, protection from wind, and an undisturbed layer of leaves are more useful than direct winter sunlight. Keep the pile away from the house foundation, sheds, play areas, and other places where rodents would create a health or property problem. Do not add food, and avoid using rodenticides or broad-spectrum pesticides nearby. Brush piles, fallen logs, standing dead trees that pose no safety hazard, and dense native vegetation can provide year-round refuge for many small mammals, birds, reptiles, amphibians, and overwintering insects.
Shrews are valuable but strictly wild garden visitors. They normally avoid people, although any frightened wild mammal may bite. Do not pick one up or dismantle an occupied shelter to examine it. Wild mammals may also carry ticks, fleas, or disease-causing organisms, so keep pets away and contact a licensed wildlife rehabilitator when an animal appears injured or unable to escape.
For the Smallest Garden Residents
As lawns, roads, buildings, and heavily managed landscapes replace natural habitat, insects—like birds and bats—often struggle to find safe places to reproduce. A wildlife-friendly yard should provide nesting space for creatures of every size.
This type of shelter is intended primarily for solitary native bees and beneficial solitary wasps. The diameter and depth of each tunnel help determine which species will occupy it.
Many native bees and solitary wasps belong to the insect order Hymenoptera. Some excavate their own burrows, but many others depend on existing openings: abandoned beetle tunnels in dead wood, hollow plant stems, cracks in stone, or soft-centered canes they can enlarge themselves.
Providing these nesting resources does not necessarily mean constructing an elaborate “insect hotel.” Preserving natural features—bare soil, standing stems, old logs, snags, and brush piles—is usually the most effective approach. Natural sites spread nests throughout the landscape rather than concentrating them in one structure, which may reduce the buildup of parasites and disease. Small, well-managed artificial nests can supplement these features when natural cavities are scarce.
These insects reward us quickly. Native bees pollinate wildflowers, fruit trees, vegetables, and many other plants, while solitary hunting wasps gather caterpillars, aphids, beetle larvae, and other small prey for their young. Most solitary species are not aggressive around people because they do not have a large colony or honey supply to defend.
Bare Ground
About 70 percent of North America’s native bee species nest underground. The simplest way to welcome them is to leave several small areas of exposed or lightly vegetated soil in sunny, well-drained parts of the yard. These patches may be level or situated on gentle slopes and can include sandy soil, loam, or firm clay; different species prefer different textures and moisture conditions.
Avoid covering every open space with bark mulch, gravel, landscape fabric, or dense turf. Even a thin mulch layer can prevent tiny bees from reaching the soil. Once a nesting area is discovered, protect it from digging, heavy foot traffic, frequent irrigation, tilling, and pesticide treatments.
A patch containing dozens of entrance holes may look like a colony, but it is often an aggregation of solitary females. Each bee excavates and provisions her own tunnel, although many individuals may choose the same favorable bank or sunny patch.
Inside Natural Cavities
Other native bees nest above ground in preexisting holes. Dead trees and branches perforated by wood-boring beetles are especially valuable and should be retained wherever they do not create a safety hazard. Large carpenter bees (Xylocopa species) can excavate galleries in weathered wood, while mason bees, leafcutter bees, resin bees, and many solitary wasps generally occupy tunnels that already exist.
Standing stems provide another important nesting resource. Native species of raspberry and blackberry, elderberry, sumac, rose, goldenrod, sunflower, and bee balm may produce hollow or pith-filled stalks. Instead of cutting every perennial to the ground, leave stems standing through winter. In spring, trim some at different heights to expose their interiors, then allow them to remain undisturbed as long as possible. Small carpenter bees may excavate the soft pith, while other species use naturally hollow stems.
Once inside a tunnel, a female constructs a row of individual brood chambers. Mason bees separate the chambers with mud. Leafcutter bees (Megachile species) cut rounded pieces from living leaves and use them to line and divide the nest. Resin bees collect sticky plant resins, while some solitary wasps build partitions from mud, chewed vegetation, or other materials. Each chamber receives an egg and enough stored food for the developing larva.
Artificial nesting blocks can be useful, but they should be small, sheltered from rain, and designed so that the nesting materials can be replaced or opened for inspection. Smooth tunnels of several diameters—approximately 3/32 to 3/8 inch—will accommodate different species. Each tunnel must be closed at the back, and the shelter should receive morning sun without overheating during the afternoon. Permanent, oversized “bee hotels” filled with materials that cannot be cleaned may eventually harbor mites, mold, parasites, and pathogens.
This compact shelter contains bundles of hollow, untreated plant stems beneath an overhanging roof. Different tunnel diameters can attract several kinds of solitary bees and wasps. Mount the shelter firmly in a visible, rain-protected location, and replace used or damaged nesting materials regularly. A structure approximately 8 × 9 × 12 inches is large enough for most home gardens.
The eastern carpenter bee excavates nests in weathered dead wood and unpainted lumber. Adults that emerge in late summer often overwinter inside old tunnels. In spring, a female enlarges an existing passage or chews a new one, then divides it into a row of brood chambers. Each chamber contains a ball of pollen and nectar and is separated from the next by a partition of chewed wood.
WHERE DO BEES NEST?
Examples of nesting sites
Many bees use broadly available nesting places, while others depend on very particular materials or structures. Most native North American bees nest underground, but others occupy hollow stems, beetle tunnels, rock crevices, or abandoned animal burrows.
| Nesting substrate | Bee genera |
|---|---|
| Bare or lightly disturbed soil | Andrena, Halictus, Lasioglossum |
| Ground beneath sparse vegetation | Bombus |
| Vertical clay banks and earthen walls | Anthophora |
| Sandy banks and well-drained slopes | Colletes, Perdita |
| Pre-existing holes about ⅛–⅜ inch wide in dead wood, hollow stems, or properly maintained nesting blocks | Osmia, Megachile, Hoplitis, Heriades, Hylaeus, Ashmeadiella |
| Pith-filled stems of raspberry, blackberry, elderberry, sumac, and similar plants | Ceratina, Hylaeus |
| Solid dead wood or unpainted, weathered lumber | Xylocopa |
| Crevices in rocks, masonry, and other sheltered cavities | Anthidium, Hoplitis, Osmia |
| Abandoned rodent burrows, old bird nests, nest boxes, leaf litter, and spaces beneath sheds or grass clumps | Bombus |
| Exposed nests made with plant resin, pebbles, or bits of soil | Dianthidium |
| Drainage holes and other small cavities in garden structures | Megachile |
Native flowering plants are essential companions to nesting habitat—and are often more valuable than an insect hotel alone. Rattlesnake master (Eryngium yuccifolium), a North American member of the carrot family, provides pollen and nectar for bumble bees and many solitary bees.
NEST BOXES
The removable front panel allows brief inspections of the nesting tubes to see whether a solitary bee or wasp has moved in. Keep checks infrequent, avoid handling occupied tubes, and replace the panel promptly.
Artificial nests for solitary bees and wasps are now sold alongside birdhouses and bat houses. The simplest designs consist of untreated wooden blocks containing smooth, dead-end tunnels of several diameters. Bundles of hollow reeds, paper tubes, or pithy plant stems closed at one end can also be used. Because North American mason bees, leafcutter bees, and beneficial solitary wasps vary in size, a range of tunnel widths may attract several species.
Mount a small nesting unit firmly about 3 to 6 feet above the ground, where it receives morning sunlight but is sheltered from rain, strong wind, and intense afternoon heat. An east- or southeast-facing wall beneath a roof overhang is often suitable. Leave occupied nests outdoors through winter so the insects emerge in response to natural seasonal temperatures; bringing them into a heated building could cause them to emerge before flowers and food are available.
Decorative “insect hotels” may contain clay, bark, pinecones, straw, or other materials, and their crevices occasionally shelter lady beetles, lacewings, spiders, or other small animals. Their most dependable occupants, however, are cavity-nesting solitary bees and wasps. Large permanent hotels can concentrate mites, parasites, and disease, so removable paper liners, replaceable reeds, or blocks that open for cleaning are safer than deep holes drilled into wood that cannot be maintained. Plastic tubes should be avoided because they retain moisture.
Natural habitat is even more valuable: leave some hollow stems standing, retain safe pieces of dead wood, and provide patches of undisturbed, well-drained soil for the many native bees that nest underground. A few small, well-maintained nesting units are generally better than one enormous hotel crowded with hundreds of tunnels.
This tunnel is occupied by one of North America’s standout native pollinators: the blue orchard mason bee (Osmia lignaria). In an observation nest, you may see the mound of pollen and nectar placed in each cell to feed the developing larva. The cells nearest the entrance are generally the last ones completed and often contain males, which usually emerge before the females farther inside.
The United States is home to roughly 4,000 species of native bees, and most bear little resemblance to the defensive, colony-dwelling insects people commonly fear. The small mason bees, leafcutter bees, resin bees, and solitary wasps that occupy nesting blocks are generally docile. Because each female cares only for her own nest rather than defending a large colony, these insects rarely sting unless they are trapped or handled.
A second kind of artificial nest, though usually less attractive to insects than ordinary reeds or wooden tunnels, has tremendous educational value: the observation nest (below). Its nesting channels are covered by a removable opaque panel, with a transparent surface beneath. By briefly lifting the cover, you can follow the construction of brood cells, the growth of larvae, the formation of cocoons, and—eventually—the emergence of the adults without opening or destroying the nest.
The materials separating the cells often reveal who has moved in. Mud partitions suggest mason bees or potter wasps. Neatly cut circles of leaves point to leafcutter bees, while plant fibers may belong to carder bees. Resin, flower petals, pebbles, or pieces of grass provide additional clues. The provisions inside are equally informative: bees leave pollen and nectar, while solitary hunting wasps stock their cells with spiders, aphids, caterpillars, crickets, or katydids.
Occasionally, the nest may also attract tiny parasitoid wasps, beetles, mites, or other organisms associated with cavity nests. These visitors are part of the natural community, but heavy infestations can harm developing bees. Use nesting tunnels that can be opened, replaced, or cleaned rather than permanent drilled blocks, and replace soiled materials regularly to prevent the buildup of parasites and disease. Smaller nesting units distributed around the garden are generally safer than one enormous “insect hotel.”
BEES OR WASPS? — A ROOM FOR EACH
These are some of the North American insects most likely to occupy a properly designed tunnel nest. The exact species will vary with region, habitat, season, and tunnel size.
| Common cavity nesters | Cell or nest material | Food supplied to young | Approximate tunnel diameter |
|---|---|---|---|
| Yellow-faced bees (Hylaeus) | Cellophane-like glandular lining | Pollen and nectar | 3/32–3/16 in. (2.5–5 mm) |
| Orchard and mason bees (Osmia) | Mud; sometimes chewed leaves | Pollen and nectar | 3/16–3/8 in. (5–9.5 mm) |
| Small mason bees (Hoplitis, Ashmeadiella) | Mud, leaf pulp, petals, or small stones | Pollen and nectar | 1/8–1/4 in. (3–6 mm) |
| Leafcutter bees (Megachile) | Cut pieces of leaves or petals | Pollen and nectar | 3/16–5/16 in. (5–8 mm) |
| Resin bees (Heriades and some Megachile) | Plant resin, fibers, and grit | Pollen and nectar | 3/32–3/16 in. (2.5–5 mm) |
| Wool-carder bees (Anthidium) | Soft plant hairs, sometimes mixed with resin | Pollen and nectar | 3/16–5/16 in. (5–8 mm) |
| Spider-hunting wasps (Trypoxylon) | Mud partitions | Paralyzed spiders | 1/8–1/4 in. (3–6 mm) |
| Aphid-hunting wasps (Passaloecus, Psenulus) | Resin, plant fibers, or glandular secretions | Aphids | 3/32–3/16 in. (2.5–5 mm) |
| Grass-carrying wasps (Isodontia) | Dry grass and other plant fibers | Crickets or katydids | 1/4–3/8 in. (6–10 mm) |
| Mason and potter wasps (Ancistrocerus, Euodynerus) | Mud partitions | Caterpillars | 3/16–5/16 in. (5–8 mm) |
Providing several tunnel diameters—from about 5/64 to 3/8 inch—will accommodate a greater variety of local species. Tunnels should be smooth, closed at the back, deep enough for several brood cells, and sheltered from moisture. Allow wild insects to colonize them naturally rather than importing cocoons from distant regions.