What Are a Bunch of Bats Called? A Colony, Cloud & More

A group of bats is most commonly called a colony, and that term dominates both scientific literature and everyday usage. But English has a fondness for colorful collective nouns, and bats have accumulated several: a cloud of bats describes them in flight, a camp refers to a large roosting aggregation of fruit bats, and a cauldron captures the swirling, funnel-like pattern bats sometimes form when leaving a cave at dusk. The word “colony” stuck for a reason, though. Unlike many fanciful animal group names that exist mainly in trivia books, “colony” reflects something real about how most bat species live: in structured, socially complex groups that persist across seasons and years.

Why “Colony” Is the Standard Term

Animal collective nouns in English range from the practical to the purely whimsical. A corpus study of written British English found that a small number of collective nouns do most of the heavy lifting: “flock,” “herd,” “pack,” and “swarm” together accounted for almost half of all uses, while hundreds of rarer terms appeared only sporadically.¹ “Colony” falls somewhere in between. It is not just a decorative label dreamed up by a medieval hunting manual. Biologists use it as a working term because bat groups genuinely share features with other animal colonies: a shared roost site, coordinated activity patterns, overlapping generations, and social relationships that persist over time.

In bat research, “colony” almost always refers to a group that shares a roost, whether that roost is a cave, a hollow tree, a bridge expansion joint, or an attic. A single colony can number anywhere from a dozen individuals to millions, depending on the species and the site. The term applies year-round but is especially prominent during the maternity season, when females gather to give birth and raise pups, and during hibernation, when bats cluster together underground.

Other Collective Nouns and When They Apply

Beyond “colony,” several other terms circulate, each capturing a different moment in bat behavior. A “cloud” of bats describes the visual impression of thousands or millions of bats in flight, particularly during the mass emergence from a large roost at dusk. The word works because the sheer density of flying bats can look like a dark, shifting cloud against the sky. Brazilian free-tailed bats emerging from cave roosts form dense columns in which adjacent bats are separated by remarkably small distances, and at large roosts housing hundreds of thousands of individuals, these columns are visible on weather radar.²

A “camp” is used primarily for flying foxes and other large fruit bats (the megabats), which roost in trees rather than caves. Flying fox camps can drape entire groves in a living canopy, with thousands of bats hanging from branches in plain sight during the day. The term has become standard in Australian English and in conservation management plans for species like the grey-headed flying fox. A “cauldron” is less formal but evocative: it describes the spinning, vortex-like flight pattern bats sometimes produce as they spiral upward out of a cave entrance. You will see it in nature documentaries more than in scientific papers.

A few other terms pop up in collective-noun lists: a “colony” or “cloud” may appear alongside “flock,” though “flock” feels borrowed from birds and is rarely used by anyone who studies bats. The practical reality is that “colony” covers the vast majority of situations, “cloud” works well for bats in flight, and “camp” is standard for flying foxes.

Maternity Colonies and Why Females Group Together

The most intensely social period in many bat species’ lives is the maternity season, roughly late spring through midsummer in temperate regions. Female bats congregate in maternity colonies to give birth and nurse their pups, and the composition and behavior of these groups have been studied in detail. Northern long-eared bats, for instance, form non-random maternity colonies under exfoliating bark or in tree cavities during May through July, meaning the females actively choose to associate with particular individuals rather than simply landing in the nearest available roost.³

In little brown myotis, researchers found that females became less likely to switch roosts once they entered the lactation period, even after accounting for weather. The shift in roost-switching behavior altered the overall group structure of the maternity colony, with network cohesiveness declining as mothers became more sedentary.⁴ This makes intuitive sense: a mother nursing a pup that cannot yet fly has strong reasons to stay put, and the colony’s social fabric reshapes itself around that constraint.

Grouping during the maternity season serves thermoregulation above all. Pups grow faster when the roost stays warm, and a cluster of adult bodies generates heat more efficiently than a solitary bat can manage. But the benefits are not unlimited. Crowding also raises the risk of parasite transmission and competition for the best roosting spots, which helps explain why colony sizes settle at different equilibria for different species.

Fission-Fusion Dynamics

If you picture a bat colony as a fixed group that sticks together night after night, the reality is more fluid than that. Many bat species follow what biologists call a fission-fusion pattern: the colony exists as a social unit, but on any given night, its members split into smaller subgroups across several roost trees or crevices, then recombine in different configurations the next day. Forest-dwelling big brown bats, tracked by radio telemetry, switched trees on average every couple of days and associated with different roostmates each time. Yet they stayed loyal to a small area of forest, and bats that switched roosts more frequently ended up maintaining associations with a larger number of individuals.⁵

This pattern looks chaotic from the outside, but it serves multiple purposes. Modeling work has shown that fission-fusion grouping can resolve a tradeoff between two conflicting needs: gathering accurate information about roost quality and minimizing the risk of infection from roostmates. By dynamically adjusting group size, bats can balance social learning against disease exposure.⁶ And the shuffling does not mean bats are socially indifferent. A five-year study of wild Bechstein’s bat colonies, drawing on over 20,000 individual roosting observations, showed that bats maintained long-term social relationships despite the constant group reshuffling.⁷ Individuals of different ages, sizes, and reproductive states kept preferring each other’s company over years, even as the specific subgroup they shared a tree with changed from day to day.

Solitary Species That Break the Pattern

Not all bats are colony animals. A meaningful minority of species are solitary roosters, and their lifestyles look nothing like the bustling maternity colonies of cave bats. The hoary bat, one of North America’s largest species, roosts alone in tree foliage. Lactating females choose roost sites that face south for sun exposure and offer shelter from wind, compensating for the lack of communal body heat by selecting microclimates that reduce the energy cost of staying warm.⁸ In the tropics, the lesser bare-backed fruit bat roosts alone in the foliage of understory trees in lowland rain forest.⁹

For solitary species, there is no colony to name. You would simply refer to an individual bat or, at most, a mother-pup pair. The distinction matters because people sometimes assume all bats live in huge cave swarms. In reality, the order Chiroptera includes over 1,400 species with an enormous range of social arrangements, from solitary foliage roosters to colonies numbering in the millions.

Mixed-Species Groups

Some roosts host more than one species at a time, and these mixed-species assemblages add another layer to the vocabulary of bat groupings. In tropical forests, researchers have documented non-random associations between species sharing the same roost, meaning certain species pairs show up together more often than chance would predict.¹⁰ The reasons may involve shared preferences for roost microclimate, mutual predator vigilance, or simply a shortage of suitable roost sites forcing species into the same spaces. These mixed groups are still called colonies in casual usage, though a researcher studying them would typically specify the species composition.

Hibernation Clusters

When temperatures drop, many temperate bat species enter hibernation in caves, mines, and other underground sites known as hibernacula. The groups that form there are sometimes called hibernation colonies, but the internal structure differs from summer maternity colonies. During hibernation, bats often cluster into tight physical huddles, and the size and placement of these clusters shift as winter progresses.

In greater mouse-eared bats, the number of solitary roosters decreased from autumn to spring, while the clusters that did form grew steadily larger: from a median of about 5 bats per cluster in November to about 20 by March. The researchers proposed that huddling helps bats reduce water loss during the driest part of winter and may synchronize the energetically expensive arousals that bats undergo periodically during hibernation.¹¹ Separate work found that as bat abundance increased in a hibernaculum, a greater proportion of individuals chose thermally stable corridors, and those that clustered together formed progressively larger groups.¹²

Bats also shift their position within the hibernaculum over the course of winter. One study found that early in the season, most bats roosted deep inside an abandoned mine where temperatures were warmer. By late winter, the majority had moved closer to the entrance where it was cooler, a shift that reduces energy expenditure during torpor. The largest huddles formed near the entrance in late winter.¹³ So a “colony” in a hibernaculum is not a static blob of sleeping bats but an actively reorganizing group responding to temperature, humidity, and the dwindling fat reserves of its members.

How Bats Navigate Dense Swarms

When thousands of bats pour out of a cave at once, the question is not just what to call them but how they avoid smashing into each other. Echolocating bats in dense emergence columns face a serious signal-processing challenge: the air is saturated with calls from neighbors, and separating your own echoes from the noise should, in theory, be nearly impossible. Yet collisions are rare.

Modeling work suggests that a bat in the center of a group can detect neighbors in groups of up to about 100 individuals, and that detection improves when bats space their calls further apart in time, use shorter call durations, and fly in more variable directions.¹⁴ But in swarms larger than that, individual echo detection becomes unreliable. A recent simulation found that bats in dense swarms may not need to distinguish individual echoes at all. Instead, they can exploit the collective soundscape, using the overall acoustic environment rather than specific echo returns to maintain safe distances from walls and from each other.¹⁵ Brazilian free-tailed bats also produce distinct call types during emergence that differ from their foraging echolocation, including constant-frequency calls that resemble social calls, hinting that the emergence column may involve communication as well as navigation.²

Eavesdropping and Group Foraging

The social benefits of bat colonies extend beyond the roost. Some species appear to hunt cooperatively, or at least benefit from foraging in the same area. Bats can eavesdrop on each other’s echolocation calls to detect prey at greater distances than they could manage alone. Because sound only has to travel one way for a bat to overhear a neighbor’s call (rather than making the round trip needed for an echo), bats can effectively “detect” insects over much larger distances by listening to changes in nearby bats’ echolocation patterns.¹⁶ This passive information transfer is similar to how flocking birds cue off each other’s behavior to find food, except that bats do it in the dark using sound rather than sight.

Famous Large Colonies

The scale of some bat colonies is hard to grasp. Bracken Cave in south-central Texas hosts what is often cited as the world’s largest bat colony, with an estimated 15 to 20 million Mexican free-tailed bats during summer. A 22-year radar monitoring study at Bracken Cave documented that the colony’s spring migration and summer reproductive cycle have advanced by roughly two weeks over the study period, likely in response to changing climate conditions. The study also documented the growth of a newly established overwintering population at the site.¹⁷ The nightly emergence from Bracken Cave is one of the natural world’s great spectacles, with the stream of bats visible on weather radar as a bloom expanding outward from the cave.

Other notable colonies include the Congress Avenue Bridge colony in Austin, Texas, where roughly 1.5 million Mexican free-tailed bats roost in the bridge’s expansion joints each summer. Researchers studying urban bridge colonies in Tucson, Arizona, found that the amplitude of bat vocalizations at a roost strongly predicted the number of bats present, offering a practical way to monitor colony size without physically counting individuals.¹⁸ In Florida alone, surveys combining random and non-random bridge sampling found 151 bridges occupied by roosting bats, underscoring how common urban bat colonies have become in regions where natural cave roosts are scarce.¹⁹

Ecosystem Services of Bat Colonies

Large bat colonies are not just impressive to watch. They perform outsized ecological work. Insectivorous colonies suppress agricultural pests, with the Brazilian free-tailed bats of central Texas alone estimated to consume thousands of tons of insects nightly. Fruit bat camps disperse seeds and pollinate plants, including economically important crops. Bats also redistribute nutrients: the guano deposited below large roosts supports entire cave ecosystems, from invertebrates to fungi.²⁰ Beyond pest control and pollination, bat colonies contribute to nutrient cycling, guano mining, and even tourism. The nightly emergence at the Congress Avenue Bridge, for example, draws hundreds of thousands of spectators each year.²¹

Disease Risk and Colony Living

The flip side of dense social living is disease transmission. Bat colonies have received intense scientific attention as potential reservoirs for viruses, and the dynamics of colony formation influence when viral shedding peaks. A study of a European bat colony found strong amplification of RNA viruses during two key periods: colony formation in spring and the weeks after females gave birth.²² DNA viruses did not show the same pattern, suggesting that the physiological stress and social mixing of these periods specifically boost RNA virus circulation. Separately, researchers have proposed two broad hypotheses for why bat populations shed viruses in pulses: either persistently infected individuals shed episodically, or transient epidemics sweep through populations as bats congregate and disperse.²³

The connection between colony structure and disease risk is one reason bat biologists pay such close attention to fission-fusion dynamics. A colony whose members constantly reshuffle may spread a pathogen faster than one where subgroups remain stable, and understanding these patterns is relevant to predicting spillover risk to other species.

Bats in Cultural Imagination

The language we use for groups of bats reflects centuries of cultural attitudes. In Western traditions, bats have long been associated with darkness, evil spirits, and vampirism, which gave rise to ominous-sounding collective nouns like “cauldron” and “plague.”²⁴ But these associations are far from universal. A review of cultural values across the Asia-Pacific region found that roughly 62 percent of the cultures examined held only positive associations with bats, linking them to good fortune, fertility, or spiritual protection. Only about 10 percent held exclusively negative views.²⁵ In Chinese culture, the word for bat is a homophone for “good fortune,” and bat imagery has been a symbol of happiness for centuries. The collective nouns that English speakers inherited lean Gothic, but they are a cultural artifact of one region’s folklore, not a reflection of the animals themselves.