Tricolored bats (Perimyotis subflavus), among the smallest bats in eastern North America, split their year between tree canopy roosts in summer and sheltered hibernation sites in winter, with a surprisingly complex migration pattern connecting the two. Once considered common across the eastern half of the continent, their populations have plummeted in recent years, making a detailed understanding of where they roost, how they forage, and where they travel more urgent than ever. What researchers have learned challenges earlier assumptions that these bats are simple homebodies with predictable habits.
Where Tricolored Bats Roost in Summer
During warmer months, tricolored bats favor roosts high in the forest canopy, tucking themselves among foliage and epiphytes. The specific roost choice varies by geography. In northern parts of the range, such as Nova Scotia, one study found that roost selection appeared to be driven by greater coverage of bony beard lichen on branches, which offers both concealment and insulation. Farther south, bats settle into clusters of dead hardwood leaves, pine needles, or Spanish moss hanging from live oaks and other trees. They have also been found roosting on bridges, in culverts, and on other human-made structures, and occasionally they linger in sites that double as overwintering locations.1Frontiers in Conservation Science. Literature review of tri-colored bat natural history with implications to management
Culverts turn out to be more important than you might expect. A survey of over 200 culverts across ten ecoregions in Texas found that tricolored bats selected culverts based on structural features like the number of internal sections, total length, and portal height, as well as environmental factors including elevation, temperature, and surrounding vegetation greenness.2Oxford Academic (Journal of Mammalogy). Structural and environmental predictors of presence and abundance of tri-colored bats in Texas culverts These human-made tunnels can serve as summer day roosts, transitional shelters during migration, and even winter hibernation spots in milder climates. The fact that bats are choosy about which culverts they use suggests that not all seemingly suitable structures are equal, and that subtle differences in microclimate inside these passages matter.
Maternity Colonies and Roost Switching
Female tricolored bats gather in maternity colonies during the breeding season, separating from males, who tend to roost alone. These colonies are notably small compared to many other bat species. Reported colony sizes range from a single female to a maximum of 75 individuals recorded in a windowless barn, with an average of around four bats per colony. The timing of birth varies with latitude: females in the southern part of the range give birth earlier than those farther north.3Frontiers in Conservation Science. Literature review of tri-colored bat natural history with implications to management
Tricolored bats do not settle into one roost and stay put all summer. Roost-site fidelity averages only about four days before an individual switches to a nearby alternative. This roost-switching behavior is common among foliage-roosting bats and likely serves multiple purposes: avoiding parasites that accumulate at a single site, tracking shifting microclimates as weather changes, and reducing the risk that a predator learns to check the same spot repeatedly. The short fidelity period also means that protecting a single roost tree is not enough; these bats rely on a network of suitable roost sites across a landscape.3Frontiers in Conservation Science. Literature review of tri-colored bat natural history with implications to management
Winter Hibernation and Torpor
As temperatures drop, tricolored bats move to hibernation sites where they spend the winter in torpor, a state of dramatically reduced metabolism and body temperature. Traditional hibernation sites include caves, mines, and rock crevices, but in the southeastern United States, where deep caves are scarce, tricolored bats regularly hibernate in culverts, under bridges, and inside storm drains. These aboveground sites tend to be thermally unstable, meaning internal temperatures fluctuate with the weather outside rather than staying nearly constant the way a deep cave does.
Research tracking skin temperatures of hibernating tricolored bats in the Southeast found that bats did enter deep torpor at these exposed sites, with torpid skin temperatures averaging around 15.7 °C and torpor bouts lasting an average of about four days, though individual bouts ranged from less than an hour to over two weeks. Males maintained significantly longer torpor bouts than females, and torpid skin temperature was closely tied to the temperature and humidity of the hibernation site itself.4Journal of Mammalogy. Winter torpor patterns of tricolored bats (Perimyotis subflavus) in the southeastern United States When bats did arouse from torpor, they warmed rapidly to body temperatures around 32 °C and stayed active for an average of roughly an hour and a half before dropping back into torpor.5Journal of Mammalogy. Winter torpor patterns of tricolored bats (Perimyotis subflavus) in the southeastern United States
At thermally unstable roosts like bridges and culverts, torpor patterns shift in response to weather. One study tracking bats at such sites found shorter average torpor bouts of about 2.7 days, with bout duration inversely related to ambient temperature: warmer nights meant shorter bouts. Bats maintained non-random arousal patterns clustered around dusk, and roughly half of all arousals included passive rewarming, meaning the bat’s body temperature rose partly due to warming air rather than entirely from internal metabolic heat. Bats were active on about a third of tracked days, and activity away from the roost increased on warmer nights and when barometric pressure dropped.6PubMed Central. Thermally unstable roosts influence winter torpor patterns in a threatened bat species This is an energy-saving trick: by letting the environment do some of the warming work, bats burn less of the fat they need to survive until spring.
Acoustic monitoring at hibernation sites confirmed that tricolored bats were active on some winter nights at every site studied, including bridges, with the highest levels of nightly activity recorded at one bridge location.7Scientific Reports. Winter activity of tricolored bats in aboveground and subterranean hibernacula in the southeastern USA These winter flights are thought to serve several purposes. Bats may drink water, forage on insects active during mild spells, or relocate to a different microsite within or near their hibernaculum. The level of winter activity at surface roosts like bridges was higher than researchers had previously assumed for a hibernating species, reinforcing the idea that tricolored bats in the Southeast lead a more dynamic winter life than their cave-hibernating relatives farther north.
Migration Patterns
For a long time, tricolored bats were assumed to be relatively sedentary, moving only short distances between summer and winter habitats. Stable hydrogen isotope analysis of bat fur changed that picture. Because the isotope signature in fur reflects the latitude where the fur grew, researchers can compare the isotope ratio in a bat’s coat to the expected ratio at the location where the bat was captured. If the two do not match, the bat moved.
Using this approach, a study found that about a third of males sampled during the non-molt period had isotope values indicating they had been captured south of where their fur grew, consistent with southward migration. Among females, roughly 16% showed evidence of having grown their fur at a more northern location, though the distances involved tended to be shorter than for males.8PLOS ONE. Evidence of Latitudinal Migration in Tri-colored Bats, Perimyotis subflavus The sex difference is striking and may relate to the different energy demands on males and females: males can afford to invest in longer migrations because they do not face the caloric burden of pregnancy and lactation that pins females closer to productive foraging grounds.
Even more unexpectedly, research at the southern edge of the species’ range revealed that some tricolored bats migrate northward in autumn rather than southward. Because roost temperatures above a certain threshold increase the energetic cost of torpor, causing bats to burn through their fat reserves too quickly, it appears that bats at the warm southern fringe travel north to find cooler hibernation sites where they can survive winter more efficiently.9PubMed Central. Tricolored bats at a southern range edge exhibit partial migration northward in autumn This is a case where being too warm is actually a survival problem: a hibernaculum that never gets cold enough forces the bat to arouse too often, each arousal costing precious fat. Moving north to a cooler site can mean the difference between making it through winter and starving before spring.
The emerging picture is one of partial migration, where some individuals in a population migrate while others stay put. This is a common strategy in birds but has been harder to document in bats. The proportion of migrants, the direction of movement, and the distances traveled all vary by sex and by where the bat lives. Populations in the middle of the range may barely move, while those near the northern or southern edges undertake meaningful seasonal journeys.
Foraging Behavior
Tricolored bats are aerial insectivores that hunt on the wing, using echolocation to find and capture small flying insects. Their echolocation calls are relatively high-frequency compared to larger bat species, which suits them for detecting small prey like moths, beetles, and flies in cluttered environments near vegetation edges. They tend to forage over waterways, along forest edges, and in canopy gaps rather than in open fields.
Average foraging distance from the roost is about 1.8 km, with a recorded maximum of roughly 4.4 km.3Frontiers in Conservation Science. Literature review of tri-colored bat natural history with implications to management That is a modest range, and it means tricolored bats depend heavily on the habitat quality within a small radius of their roost. A roost surrounded by dense forest with no water features or edge habitat may not provide enough foraging opportunity, even if the roost itself is structurally ideal. This tight link between roosting and foraging habitat makes landscape-level conservation especially important for the species: it is not enough to protect roost sites if the surrounding foraging grounds are degraded.
The small body size of tricolored bats, typically weighing between 4 and 8 grams, constrains their foraging strategy. They cannot carry large fat reserves the way bigger species can, which makes nightly foraging success more critical and makes them more vulnerable to stretches of bad weather that ground insect prey. During summer, females supporting pregnancy or nursing pups face even tighter energy budgets, which may explain why maternity colonies tend to form in areas with reliable insect-rich foraging habitat nearby.
White-Nose Syndrome and Behavioral Shifts
No discussion of tricolored bat ecology is complete without addressing white-nose syndrome (WNS), the fungal disease that has devastated hibernating bat populations across North America since it was first detected in 2006. The fungus, Pseudogymnoascus destructans, grows on the skin of bats during hibernation, disrupting their torpor cycles and causing them to arouse too frequently, which burns through fat reserves and leads to starvation or dehydration before spring.
At one well-monitored hibernation site, the number of tricolored bats dropped by about 90% during the first three years after WNS arrived. Before the disease hit, nearly all the bats in this tunnel hibernaculum clustered in the warmest sections at the back. After WNS, a significant shift occurred: bats increasingly moved to the colder front portions of the tunnel, particularly during the period when population numbers began to stabilize and slightly recover from 2018 to 2021.10PubMed Central. Changes in hibernating tricolored bat (Perimyotis subflavus) roosting behavior in response to white-nose syndrome The researchers proposed that this behavioral shift may have helped surviving bats, because the WNS fungus grows more slowly in colder temperatures, and cooler roost spots also reduce the metabolic cost of torpor. In other words, the bats that chose less comfortable but colder spots may have been the ones that survived, either because the fungus grew more slowly on them or because they simply burned less fat over winter.
This behavioral response is cautiously encouraging, but it does not mean the species is out of danger. A 90% population crash leaves very little margin for error, and the remaining bats face ongoing exposure to the fungus every winter. The species was proposed for listing under the U.S. Endangered Species Act in large part because of WNS-related declines, and current population levels remain far below historical baselines across most of the range.
How Researchers Track Tricolored Bats
Studying a small, nocturnal, tree-roosting bat presents obvious challenges. Two major tools for monitoring tricolored bats are acoustic detection, which records their echolocation calls, and stable isotope analysis of fur for migration studies. Both have significant limitations that are worth understanding if you encounter tricolored bat data in conservation reports.
Acoustic monitoring relies on automated software that classifies echolocation calls to species. A comparison of several widely used classification programs found that performance varied considerably depending on the species and the software. For tricolored bats specifically, one program achieved a sensitivity of 0.68, meaning it correctly identified about two-thirds of tricolored bat recordings but missed the rest. Other programs performed better for this species, exceeding 0.80 accuracy across multiple metrics.11PLOS ONE. Automated echolocation classifiers vary in accuracy for northeastern U.S. bat species The practical consequence is that different monitoring programs can produce meaningfully different estimates of tricolored bat activity at the same site, depending on which software is used.
Detection becomes even harder when tricolored bats are rare at a site. An analysis of how many call files were needed to confirm a species’ presence found that requirements differed between software programs. One program needed about ten files to detect tricolored bat presence, while another needed only six. Both programs performed poorly when a species made up less than a quarter of total bat activity at a site.12PLOS ONE. Maximum likelihood estimators are ineffective for acoustic detection of rare bat species For a species in steep decline, where bats may represent a shrinking fraction of calls at any given location, this is a real problem. Monitoring programs that rely on a single software classifier risk underestimating where tricolored bats still persist, which can lead to conservation resources being directed to the wrong places.
Why Aboveground Hibernation Sites Deserve More Attention
Historically, bat hibernation research focused on caves and mines because those sites are easy to access for surveys: you walk in, count bats on the walls, and walk out. Tricolored bats complicate that picture because a substantial portion of the southeastern population hibernates in places like bridges, culverts, and similar structures that do not show up in traditional cave surveys. The thermal instability of these sites, combined with the finding that bats are active on a meaningful fraction of winter nights, means that management strategies designed around deep-cave hibernation may not translate well to the Southeast.
Bridge replacement and culvert repair projects, for example, can destroy hibernation sites that wildlife managers did not know existed. Standard environmental reviews for infrastructure projects have not always included bat surveys of these structures, though awareness is growing. The finding that tricolored bats are selective about which culverts they use, favoring specific structural and environmental features, suggests that replacement structures could potentially be designed to maintain suitability if those features are understood and incorporated. Whether transportation agencies will consistently do this remains an open question, but the ecological case for it is becoming harder to ignore as the species’ conservation status worsens.