Bats do not build nests. This is one of the most widespread misconceptions about them, likely reinforced by the phrase “bat’s nest” appearing in casual conversation and even pest-control marketing. What bats actually have are roosts: places they hang, rest, raise pups, and hibernate, but which they do not construct, weave, or furnish with gathered material. A bat’s “nest” is just whatever surface the bat is clinging to, whether that is a cave ceiling, a gap behind peeling bark, or a crevice in your attic. Understanding what these roosts actually look like, and why bats pick one spot over another, clears up a lot of confusion about bat behavior and what to do if you find bats in or near your home.
Why Bats Do Not Have Nests
Birds build nests. Squirrels build dreys. Beavers engineer lodges. Bats do none of these things. With a few exceptions in the tropics (certain species modify large leaves by biting along the midrib to fold them into tent-like shelters), the vast majority of the world’s roughly 1,400 bat species simply find an existing space and move in. They add nothing to it. Animal architecture ranges widely in function, but shelter is by far the most common purpose, and bats have evolved to exploit ready-made shelter rather than create it.1PubMed Central. Roost making in bats What you see at a bat roost is simply the bats themselves, clustered or solitary, gripping a surface with their feet, sometimes surrounded by the unmistakable dark stain and crumbly pellets of accumulated guano. There is no bedding, no woven material, no collected debris.
This matters practically. If you find a pile of droppings in an attic or under an eave and suspect bats, you will not find a nest to remove. The “nest” is the structural gap itself. Bats are using the architecture of the building, the tree, or the rock formation as-is.
What Cave and Underground Roosts Look Like
When most people picture bats, they picture them in caves, and for good reason. Caves, mines, and culverts provide the stable temperatures and high humidity that many bat species need, particularly during hibernation. A bat roost inside a cave is not a discrete structure you could point to; it is a stretch of ceiling or wall where bats cluster together, sometimes by the hundreds or thousands, hanging upside down by their toes. Below them, guano accumulates in mounds that can grow surprisingly deep over decades. In well-established caves, these guano piles support entire arthropod communities, from beetles and mites to specialized cave-adapted flies, all feeding on the nutrient-rich droppings.2PubMed Central. Bat Colony and Cave Zone Shape Arthropod Assemblages in Levantine Caves
The microclimate inside a cave roost is far more stable than conditions outside. Research on tri-colored bats roosting in both caves and culverts found that temperature and humidity at the roost site were primarily influenced by outside temperature and how far the roost was from the entrance. Deeper sites stayed more buffered from outdoor swings, while spots near entrances tracked ambient conditions more closely.3PubMed Central. External temperature and distance from nearest entrance influence microclimates of cave and culvert-roosting tri-colored bats (Perimyotis subflavus) This is why bats are picky about exactly where inside a cave they settle. Two spots fifty meters apart can offer very different conditions. Species that roost in hotter parts of caves have evolved substantially higher heat tolerance, able to handle ambient temperatures roughly 4°C higher than cool-roosting species before losing coordinated movement.4Functional Ecology. Caves, crevices and cooling capacity: Roost microclimate predicts heat tolerance in bats
From a visual standpoint, an active bat cave roost looks like a living, shifting ceiling. The bats are packed tightly, fur touching fur, occasionally shuffling. The walls and ceiling near the roost zone are typically stained dark from body oils. The floor below is layered with guano, and the air has a sharp, ammonia-tinged smell. During winter hibernation the scene is quieter: bats hang motionless, sometimes frosted with condensation, their metabolic rate dropped to a fraction of normal.
What Tree Roosts Look Like
Many bat species never set foot (or wing) in a cave. Forest bats roost in tree cavities, behind loose bark, in the dead crowns of snags, or in dense foliage. These roosts are invisible to casual observers. You would walk past a roost tree without knowing it, because the bats are tucked into narrow crevices or internal hollows that show no outward sign of use beyond a faint dark smudge near the opening.
Female big brown bats in North American forests, for example, preferentially select trees with multiple holes or long crevice openings over trees with a single small hole. The reason is simple: bigger cavities hold bigger groups. Roosting-group size correlates with cavity volume, and colony-forming species need space.5Journal of Mammalogy. Roost Selection by Forest-Living Female Big Brown Bats (Eptesicus fuscus) In Australian timber-production landscapes, different species show strikingly different preferences: one species favors old, highly decayed trees along stream buffers and switches roosts daily, while another gravitates to logged areas and may stay in a single roost for over a month.6PubMed Central. Interspecific differences and commonalities in maternity roosting by tree cavity-roosting bats over a maternity season in a timber production landscape
If you were to peer inside one of these tree roosts, you would see bats wedged into crevices, sometimes stacked on top of one another, clinging to the rough interior wood. There is no lining, no insulation added by the bats. The roost is the tree’s own architecture: a split, a wound, a woodpecker hole, or a hollow trunk.
Bats in Buildings, Bridges, and Other Human Structures
About 40% of the world’s bat species are capable of roosting in human-made structures, a figure that surprised researchers when it was recently quantified at a global scale.7iScience. Global determinants of anthropogenic roosting in bats Bridges, attics, barns, mines, drainage pipes, and the overhangs of buildings all serve as roosts. Species that use human structures tend to be habitat generalists with large geographic ranges, small body sizes, and insect-based diets. Anthropogenic roosting is not a recent behavioral accident; it shows a moderate evolutionary signal, meaning certain lineages have been gravitating toward built structures (or their natural analogs) for a very long time.
What does a bat roost in a building actually look like? Typically it is a gap you might barely notice: the space between roof tiles and the underlying boards, the narrow void behind a fascia board, or the interior of a wall cavity accessed through a crack as narrow as a finger’s width. Inside, you will find the bats hanging from or pressed against the surface, with guano and urine staining accumulating below. In warm attic spaces, colonies can number in the hundreds, clustered together in a dark mass on the highest point of the roof interior. The staining is usually the first evidence homeowners notice: dark streaks near an entry point, and a growing pile of small, rice-grain-sized droppings on the floor or ledge below.
Concrete bridges offer some of the most stable roost conditions outside of deep caves. A year-long monitoring study of a cast-in-place concrete bridge in New Mexico found that interior expansion joints above piers had minimal temperature variation compared to the outdoors, and nighttime temperatures within the concrete were warmer than ambient, while daytime temperatures stayed about 2 to 3°C cooler. The bridge roosts provided conditions reminiscent of natural cave roosts: stable temperature, high humidity, protection from weather and predators, with the added benefit of permanence.8Transportation Research Board. The Thermal Environment of a Concrete Bridge and Its Influence on Roost Site Selection By Bats (Mammalia chiroptera)
Comparisons between anthropogenic and natural roosts broadly show differences in roost temperature, bat behavior, and even bat health and parasite load, though relatively few studies have made direct side-by-side comparisons.9PubMed Central. Impacts of bat use of anthropogenic structures on bats and humans From a human perspective, the main effects of bat presence in buildings are the accumulation of guano and the potential presence of microorganisms of public-health interest.
Maternity Roosts and Why Temperature Matters So Much
During the breeding season, female bats of many species gather in maternity colonies, sometimes numbering hundreds or thousands, to give birth and raise pups. These maternity roosts look physically identical to other roosts: there is nothing added or constructed. But they are chosen with extreme precision for one thing above all: warmth. Warm roosts accelerate pup development during gestation and nursing. Later, once pups are weaned, females shift to cooler roosts where they can enter torpor and conserve energy.10Ecological Solutions and Evidence. Design modifications affect bat box temperatures and suitability as maternity habitat
This is why south-facing attic walls, sun-baked bridge joints, and dark-colored bat boxes are disproportionately used by maternity colonies. The warmth is not incidental; it is the whole point. A maternity roost in an attic during summer will be noticeably hot to a human entering the space, sometimes exceeding 40°C. Pups are visible as smaller, pinkish bats clinging to their mothers or to the roost surface nearby, often in tight clusters that help retain heat.
Why Bats Switch Roosts So Often
One of the more surprising aspects of bat roosting behavior is how frequently many species abandon a perfectly good roost and move to a new one. This is not disorientation or restlessness. For many species, roost switching is a deliberate strategy to manage parasites. Bat roosts accumulate ectoparasites: bat bugs, bat flies, ticks, and mites that wait in crevices for the bats to return. By rotating among several roosts, bats can stay ahead of parasite buildup.
A nine-year study of female Bechstein’s bats found that the bats tracked the life cycle of their parasites and avoided returning to roosts when highly contagious parasite stages were likely to be present. Their roosting decisions matched the age-dependent emergence probability of fly puparia left behind from earlier visits.11PubMed. Roost selection and roost switching of female Bechstein’s bats (Myotis bechsteinii) as a strategy of parasite avoidance A complementary experiment with pipistrelle bats found that when researchers removed all bat bugs from bat boxes each spring, the bats used those boxes to breed, something they had avoided doing for the previous eight years when the boxes were infested.12PubMed. Recolonization of bat roost by bat bugs (Cimex pipistrelli): could parasite load be a cause of bat roost switching?
This means that if you see bats in your attic one week and they are gone the next, they have not necessarily been “evicted” by anything you did. They may simply be rotating through their roster of roost sites and will circle back when enough time has passed for parasite loads to drop. Species vary widely in how often they switch. Some Australian bats move every single day, while others stay put for over a month, as noted in the tree-roosting studies above.
Night Roosts Versus Day Roosts
Bats sometimes use different roosts for different purposes within the same night. Day roosts are where they sleep and spend the daylight hours. Night roosts, by contrast, are temporary rest stops used between foraging bouts after dark, often located closer to feeding areas. Night roosts tend to be sheltered overhangs, porches, bridge undersides, or shallow cave entrances where bats hang for an hour or two to digest before heading back out.
Research at caves and mines in the Appalachian Mountains found that bats arrived at night roosts in a specific pattern, with very few captured exiting during the first hour after sunset and peak activity occurring three to five hours after dark, with minimal differences between sexes or age groups.13Oxford Academic. Nightly, Seasonal, and Yearly Patterns of Bat Activity at Night Roosts in the Central Appalachians Visually, a night roost is even less conspicuous than a day roost. There may be a few bats hanging under an overhang with small amounts of fresh guano below, but because the bats only use the site for brief rest periods, the staining and droppings never build up the way they do at a long-term day roost or maternity colony.
Scent, Staining, and the Social Life of a Roost
A bat roost is not just a dormitory. For many species, it is a social arena where mating, territorial disputes, and group bonding all happen. One of the most visible (or rather, smellable) signs of this social activity is scent marking. Male greater sac-winged bats mark their roost territories with secretions from facial glands, concentrating their marks at territory boundaries rather than where females cluster. They mark most frequently in the afternoon after females have departed, fanning wing-sac volatiles toward rival males, essentially posting a chemical “keep out” sign.14Ethology. Temporal and Spatial Distribution of Male Scent Marks in the Polygynous Greater Sac‐Winged Bat
In greater spear-nosed bats, males that defend harems have measurably different chemical profiles from bachelor males roosting in all-male groups, and individual identity can be distinguished from the scent signal alone.15PubMed. Male Scent Gland Signals Mating Status in Greater Spear-Nosed Bats, Phyllostomus hastatus The practical result is that established bat roosts carry a strong, characteristic odor that persists even when the bats are absent. This smell is a combination of guano, urine, and these glandular secretions, and it serves as a homing signal. Bats returning from foraging can smell their roost from a distance.
That persistent odor also explains why bats return to the same buildings year after year, even after exclusion work. If the scent signature remains, new bats may investigate the site. Professional bat exclusion typically involves sealing all entry points while leaving one-way exits, but the lingering odor can attract scouts for several seasons.
How Bats Find Their Way Back
Given that many species use multiple roost sites and forage kilometers from home each night, how do bats reliably return? Echolocation plays a larger navigational role than previously appreciated. A study that translocated wild Kuhl’s pipistrelle bats several kilometers from their home roost, while selectively blocking their vision, magnetic sense, or olfaction, found that bats could navigate home using echolocation alone. They appeared to build and use an acoustic cognitive map of their environment, recognizing landscape features by their echo signatures. Navigation improved when vision was also available, but echolocation by itself was sufficient for multi-kilometer homing.16PubMed. Acoustic cognitive map-based navigation in echolocating bats
This finding matters for understanding roost fidelity. Bats do not stumble onto roost sites randomly. They maintain detailed spatial knowledge of their home range and can return to specific roosts with precision, even after being experimentally displaced. It also means that a bat colony removed from a building may attempt to return to it repeatedly, guided by both acoustic landmarks and the scent cues mentioned above.
Predation Risks and What Guano Gives Away
A bat roost is not perfectly safe. Predators, including snakes, owls, hawks, and feral cats, learn to watch roost exits at dusk for an easy meal. Research using artificial roosts in urban bushland found that predators visited roosts with bat feces significantly more often than clean control roosts. Interestingly, roosts with small amounts of guano, mimicking a solitary bat, attracted the most predator visits, suggesting that bats roosting alone face a greater likelihood of disturbance or predation than those in groups.17PubMed Central. Odour cues influence predation risk at artificial bat roosts in urban bushland This is one of the reasons colonial roosting is so common among bats: there is safety in numbers, and the dilution effect means any individual is less likely to be the one caught.
Visually, predation signs at a roost can include scattered wing fragments on the ground below the entrance. Owls and hawks strip the wings and eat the body, leaving papery wing membranes as evidence. If you find bat wings beneath a tree or building edge, a predator is likely working the exit.
What Bat Boxes Look Like and Whether They Work
Given that bats do not build their own shelter, humans sometimes build it for them, both for conservation purposes and to attract bats away from buildings. Bat boxes come in several designs, and how well they work depends heavily on temperature. In Mediterranean climates, experiments with white, grey, and black bat boxes showed that black boxes most closely matched the warm temperatures bats experienced in building roosts, and they attracted the largest numbers of bats. On the hottest days, however, bats abandoned even the black boxes, suggesting that a wider internal temperature range would be ideal.18Biological Conservation. Influence of temperature in roost selection by Pipistrellus pygmaeus (Chiroptera): relevance for the design of bat boxes
Among different box styles, bats show a clear preference for rocket boxes, which are tall, multi-chambered structures with wide temperature gradients inside, over single-chamber bat boxes and bark-mimic designs. Rocket boxes offer the most roosting surface area, the largest volume, and the greatest proportion of time spent within the bats’ preferred temperature range.19PLoS ONE. In artificial roost comparison, bats show preference for rocket box style A rocket box looks like a tall, narrow wooden tower, usually mounted on a pole, with internal partitions creating multiple narrow chambers that bats can move between to find their ideal temperature. From the outside, it looks nothing like a birdhouse; it is taller, slimmer, and has a slit entrance at the bottom rather than a round hole.
If you are considering installing a bat box, the evidence points toward dark-colored, multi-chambered designs placed in sun-exposed locations, ideally near a water source. Occupancy often takes a year or more; bats need to discover the box through their nightly foraging flights, and the scent of previous occupants helps attract new ones.
Self-Maintenance at the Roost
Because bats add nothing to their roost, all maintenance effort goes toward their own bodies. Observations of captive long-nosed bats identified 18 distinct behaviors at the roost, with the most frequent being wing stretching and self-grooming.20Acta Chiropterologica. Roosting behaviour and time budgets of the Curaçaoan long-nosed bat, Leptonycteris curasoae (Phyllostomatidae, Glossophaginae) in captivity Bats groom meticulously, licking their fur and membrane surfaces to manage ectoparasites and keep their flight membranes supple. If you watch a roosting bat for any length of time, you will see it periodically unfurl a wing, stretch it, and run its tongue along the membrane. This grooming is one of their primary defenses against the parasites that accumulate at roost sites, working alongside the roost-switching strategy described above.
Social grooming also occurs in many species, particularly in maternity colonies. Mothers groom pups extensively, and in some species, unrelated females groom each other, a behavior thought to reinforce social bonds within the colony. The roost, then, is not just a place to sleep. It is where bat social networks are built and maintained, where pups learn to fly, and where the nightly cycle of emergence, foraging, return, and rest plays out across seasons and years.