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Communal roosting

Communal roosting is a social behavior observed across diverse animal taxa, defined as the aggregation of more than two individuals resting together in close proximity during periods of inactivity, such as nighttime for diurnal species or daytime for nocturnal ones.^[1] This phenomenon is most extensively documented in birds, where it manifests in groups ranging from small clusters of a few individuals to enormous assemblages numbering in the millions, but it also occurs in mammals like bats and insects such as butterflies.^[2]^[3]^[4] The adaptive benefits of communal roosting primarily revolve around survival and efficiency enhancements, including a reduction in predation risk through mechanisms like the dilution effect—where per capita attack rates decrease in larger groups—and heightened collective vigilance.^[4]^[1] Energy conservation is another key advantage, achieved via thermoregulation benefits from huddling or clustering, which lowers individual heat loss in cold conditions, particularly in birds and bats.^[2]^[3] Additionally, roosts often function as information centers, enabling individuals to observe and learn from others about ephemeral food sources, predator locations, suitable travel companions, or even potential mates, thereby boosting foraging success and social coordination.^[1] These functions can vary by species, season, and environmental pressures, with roost fidelity sometimes shifting based on resource availability or threats.^[2] Notable examples illustrate the scale and diversity of communal roosting; in birds, the extinct passenger pigeon (Ectopistes migratorius) formed colossal roosts spanning square miles with billions of individuals, while modern species like European starlings (Sturnus vulgaris) and red-billed queleas (Quelea quelea) gather in flocks exceeding a million for nightly rest, often near urban areas or reed beds.^[2] Among insects, passion-vine butterflies of the genus Heliconius roost in small groups averaging about four to five adults, leveraging collective warning coloration to deter predators like birds and ants.^[4] In mammals, neotropical bats such as Glossophaga commissarisi and Carollia sowelli share tree cavities or artificial structures in mixed-species groups, benefiting from reduced competition and shared roost stability in tropical understory habitats.^[3] Evolutionarily, communal roosting likely arose independently multiple times, with losses in some lineages due to territoriality or solitary lifestyles, underscoring its role in balancing social costs and gains.^[5]

Overview

Definition

Communal roosting is defined as the aggregation of more than two unrelated conspecific individuals that congregate in a shared location to spend the diurnal or nocturnal resting period together, typically in response to environmental cues such as dusk for diurnal species or tidal cycles for certain aquatic or semi-aquatic animals, with the assembly lasting from several hours to overnight.^[5]^[6] This behavior emphasizes a temporary, non-nesting congregation focused solely on resting or sleeping, distinguishing it from solitary roosting, where individuals rest independently; diurnal groupings, which occur during active foraging or social periods; and cooperative breeding, which involves shared parental care and offspring provisioning rather than mere overnight assembly.^[5]^[7] Roost sizes in communal roosting exhibit considerable variability, spanning small groups of 5–10 individuals to enormous aggregations of thousands or even millions, influenced by species-specific traits, seasonal changes, and local resource dynamics. For instance, some songbirds form modest roosts of dozens, while species like European starlings (Sturnus vulgaris) or Brazilian free-tailed bats (Tadarida brasiliensis) can assemble in the millions at peak times.^[6]^[4] A hallmark of communal roosting is site fidelity, whereby individuals or groups repeatedly return to the same location across multiple nights or seasons, often selecting sheltered sites such as trees, caves, rock crevices, or anthropogenic structures like buildings and bridges for protection and stability. Although documented across a broad range of taxa—including birds, bats, mammals, and insects—communal roosting has been most intensively studied in birds, where it manifests in diverse ecological contexts.^[6]^[8]^[9]

Patterns and Prevalence

Communal roosting is a widespread behavior observed in over 200 bird species across at least 28 families, occurring globally in both temperate and tropical regions, though it is more prevalent among migratory and wintering species during non-breeding seasons.^[5] In temperate zones, such as North America and Europe, roost formations often peak in late summer and fall, coinciding with post-breeding aggregation and preparation for migration, as seen in species like American robins (Turdus migratorius) and European starlings (Sturnus vulgaris), where group sizes can increase from small clusters to thousands of individuals.^[10] Tropical examples include Neotropical migrant songbirds, such as the northern parula (Setophaga americana), which form roosting aggregations on wintering grounds in regions like Jamaica, highlighting the behavior's extension beyond seasonal cold stress into resource-variable environments.^[11] Overall, prevalence is higher in non-breeding periods.^[12] Environmental triggers for communal roosting include diurnal light cycles, temperature declines, and localized weather patterns, which prompt birds to aggregate at dusk for synchronized arrival at roost sites.^[13] Site selection is influenced by factors such as shelter from wind and rain, often in dense foliage or structures, and proximity to foraging areas to minimize energy expenditure on commutes; for instance, shorebirds like dunlins (Calidris alpina) select tidal flats based on high-tide inundation and low disturbance, while ravens (Corvus corax) favor elevated sites during cold snaps for microclimate buffering.^[14]^[13] In coastal ecosystems, tidal cycles act as a key trigger, drawing waders to predictable roost locations to avoid flooding.^[15] Variations in communal roosting encompass fixed versus nomadic strategies, mixed-species versus single-species assemblages, and adaptations to urban versus natural habitats, with most patterns being nocturnal though some diurnal elements occur in crepuscular species. Fixed roosts, used consistently across seasons, are common in stable environments like urban parks for species such as Torresian crows (Corvus orru), where hundreds aggregate nightly with seasonal peaks in autumn.^[16] Nomadic roosting involves site switching, as documented in yellow-crested cockatoos (Cacatua sulphurea) in urban Hong Kong, where birds shift from green-space-adjacent sites in spring to illuminated, less-vegetated areas in summer and warmer human-proximate locations in winter.^[17] Mixed-species roosts are frequent in corvids and vultures, such as large-billed crows (Corvus macrorhynchos) and house crows (Corvus splendens) sharing trees influenced by local food availability.^[18] In natural habitats, roosts emphasize undisturbed woodlands or wetlands, whereas urban settings show increasing prevalence, with 2020s studies revealing vulture roosts on communication towers near garbage sources in suburban South Carolina, providing buffered access to anthropogenic resources despite higher disturbance.^[19]

Evolutionary Hypotheses

Information Center Hypothesis

The Information Center Hypothesis posits that communal roosts function as hubs where birds exchange information about the locations of food resources, particularly in environments with unpredictable and patchy foraging opportunities. Proposed by Peter Ward and Amotz Zahavi in their 1973 paper, the hypothesis suggests that individuals who successfully locate food patches—termed "prospectors"—return to the roost, where unsuccessful foragers, or "scroungers," can observe and follow them during the subsequent morning departure to exploit those sites.^[20] This passive transfer of information enhances overall foraging efficiency without requiring active signaling, as prospectors benefit from reduced competition through dilution of effort in locating new patches, while scroungers gain access to reliable food sources.^[21] The mechanism relies on indirect cues at the roost, such as the behavior, timing, or flight direction of returning prospectors, which informed individuals may use to join specific foraging groups the next day. This process is most adaptive for species facing ephemeral food distributions, like insects or seeds that vary daily in availability, allowing roost size to scale with resource unpredictability to optimize information flow. Mathematical models have demonstrated the efficiency of this transfer, showing that larger roosts increase the probability of scroungers locating high-quality patches while minimizing search costs for prospectors.^[22] Recent tests, such as a 2024 study on ravens, indicate that co-roosting can substantially increase naïve individuals' chances of discovering known food patches.^[23] Supporting evidence comes from experimental studies on red-billed quelea (Quelea quelea), where naive birds isolated from food but roosting with experienced ones showed significantly higher success in locating baited patches by following departing flocks, compared to controls without access to informed companions.^[24] Similarly, observations of European starlings (Sturnus vulgaris) at communal roosts have revealed patterns of synchronized departures toward known food sites, correlating with improved daily intake rates for late joiners, though site fidelity to diurnal feeding areas tempers the reliance on roost-based information.^[25] Despite this support, the hypothesis has faced criticisms for lacking direct observational proof of following behavior in many species, with radiotracking studies on starlings indicating strong individual loyalty to fixed feeding territories that reduces the need for roost-derived information.^[26] prompting alternative explanations such as active recruitment signals rather than passive observation, though experiments continue to refine the model's applicability across taxa.

Recruitment Center Hypothesis

The Recruitment Center Hypothesis posits that communal roosts serve as hubs where informed individuals actively recruit others to ephemeral food sources, enhancing group foraging success through deliberate signaling rather than passive observation. Developed by Richner and Heeb, this idea emphasizes that successful foragers return to the roost and use honest signals to assemble foraging parties, benefiting from the advantages of group exploitation of patchy resources while ensuring signal reliability to avoid exploitation by unsuccessful individuals. This contrasts with the passive information sharing in the Information Center Hypothesis, where naïve birds simply follow informed ones without active recruitment. Mechanisms of recruitment typically involve vocal or visual cues at the roost to coordinate departure and direct groups to food patches, particularly advantageous in colonial or semi-colonial species facing unpredictable foraging opportunities. In cliff swallows (Petrochelidon pyrrhonota), for instance, individuals emit specific "squeak" calls upon detecting swarming insects, attracting conspecifics to join foraging flocks and increasing the likelihood of tracking transient prey; playback experiments demonstrated that these calls elicit recruitment responses, especially under poor feeding conditions.^[27] Such active signaling parallels recruitment in social insects and is predicted to evolve when group foraging yields higher per capita intake than solitary efforts. Supporting evidence includes observations in cliff swallows, where larger recruited groups correlated with improved foraging efficiency due to better insect tracking, and experimental manipulations confirming that call playback boosts group assembly and success rates. In common mynas (Acridotheres tristis), roost aggregations were hypothesized to facilitate similar vocal recruitment to fruit patches, though field tests showed limited support, with group sizes not strongly predicting foraging gains. Recent studies on European starlings (Sturnus vulgaris) link seasonal roost switching to proximity of high-quality foraging sites, suggesting recruitment dynamics adapt to resource shifts, thereby maintaining efficiency in dynamic environments. The Social Refuge-Territory Prospecting Hypothesis posits that communal roosts function as safe havens for socially subordinate nonbreeding individuals, allowing them to avoid aggression from dominant territory holders while prospecting for future breeding opportunities. Proposed by Dwyer, Fraser, and Morrison in 2018, this hypothesis emphasizes roosts as central locations from which nonbreeders can gather and store information about high-quality territories without incurring immediate competitive costs. In species like the crested caracara (Caracara cheriway), roost use enables these individuals to balance survival needs with long-term reproductive prospects, particularly in habitat-limited environments where breeding territories are scarce.^[28] Key mechanisms underlying this hypothesis include the storage of territorial information as memory rather than direct energy reserves, facilitating informed breeding attempts in subsequent seasons. Nonbreeders engage in social learning by observing cues of territory productivity and holder vulnerability during prospecting forays from the roost, which is especially relevant for seasonal breeders facing temporal constraints on territory acquisition. For instance, in seasonal species, roosts provide a low-risk base during nonbreeding periods when competition is relaxed, allowing energy allocation toward survival and future reproduction without the risks of solitary foraging or dispersal. This contrasts with voluntary aggregation for foraging, focusing instead on involuntary refuge use driven by social hierarchies.^[28] Empirical evidence from raptor studies supports this framework, particularly observations of subadult and nonbreeding crested caracaras in Florida, where communal roost attendance was significantly higher during nonbreeding seasons (mean of 111.8 individuals per night) compared to breeding seasons (mean of 60.7 individuals per night), based on 407 nightly counts from August 2006 to April 2009. Modeling approaches demonstrate fitness gains from delayed breeding, showing that nonbreeders optimize long-term reproductive success by using roosts to minimize exploratory risks while maximizing survival probabilities through differential survival data. The hypothesis integrates elements from the Information Center Hypothesis by extending information-sharing benefits to territory prospecting rather than solely foraging, though it highlights roost use as a refuge strategy for subordinates precluded from breeding.^[28]

Benefits

Communal roosts function as hubs for information exchange among birds, enabling individuals to reduce search times for food by observing and following conspecifics that have located profitable patches. Uninformed birds can join successful foragers during synchronized departures, a mechanism that is especially advantageous in patchy, unpredictable environments where solitary prospecting carries high energetic costs. This shared knowledge promotes group foraging, which further enhances efficiency by allowing collective exploitation of resources while distributing the risks of exploration across the group.^[21]^[1] Empirical evidence from the red-billed quelea (Quelea quelea) illustrates these benefits, as birds exhibit highly synchronized dawn departures from primary roosts to known feeding grounds based on the prior day's outcomes, covering areas up to 5,000 km². Secondary day-roosts allow mid-day adjustments to new patches by permitting naïve individuals to follow informed ones, resulting in markedly improved foraging success following suboptimal feeding periods. In European starlings (Sturnus vulgaris), communal roosting similarly supports information transfer on distant or ephemeral food sources, with phylogenetic analyses indicating that such behavior evolved alongside flocking to boost efficiency in locating rich patches through companion use.^[21]^[5] Theoretical models of social foraging highlight an optimal group size for information transfer, where gains from pooled knowledge balance against increasing interference and competition. For instance, simulations for scavengers like black vultures show peak foraging success in groups of 3–5 individuals, with larger roost assemblages exceeding this size to facilitate broader information dissemination beyond optimal foraging units. Recent microhabitat studies on yellow-crested cockatoos (Cacatua sulphurea) in 2021 revealed that roost selection in spring favors sites with high canopy cover near urban parks—key foraging patches—demonstrating how proximity minimizes travel costs and amplifies the role of roosts in enhancing daily food acquisition.^[29]^[17]

Thermoregulation and Energy Conservation

Communal roosting in birds facilitates thermoregulation by enabling huddling, which decreases the collective exposed surface area to the environment and thereby minimizes convective and radiative heat loss.^[30] This behavioral adaptation is particularly effective in cold conditions, as clustered individuals create a warmer microclimate within the roost, reducing the temperature gradient between body core and ambient air.^[31] Studies on small passerines, such as the scaly-feathered finch (Sporopipes squamifrons), demonstrate that groups of eight to twelve birds can lower their resting metabolic rate (RMR) by over 30% compared to solitary individuals when roosting without insulation, with even greater savings—up to 50%—in insulated nest structures that further trap heat.^[32] These mechanisms collectively reduce energy expenditure for maintaining homeothermy, allowing birds to allocate resources toward survival and reproduction rather than constant heat production.^[33] Empirical evidence from respirometry measurements supports these physiological benefits, showing decreased oxygen consumption in communally roosting birds during rest phases. For instance, in green woodhoopoes (Phoeniculus purpureus), oxygen uptake was significantly lower in groups of five compared to solitary or trio roosts, with reductions scaling negatively with group size due to shared body heat.^[34] Similarly, infrared thermography applied to roosting birds reveals heat sharing patterns, where peripheral individuals experience elevated surface temperatures from adjacent companions, confirming reduced individual heat dissipation.^[35] In very small species like the verdin (Auriparus flaviceps), communal roosting in cavities lowered thermal conductance by insulating the group against external cold, with oxygen consumption dropping proportionally to cluster density.^[36] The thermoregulatory advantages of communal roosting are especially pronounced in small-bodied and wintering species, which face high surface-to-volume ratios and elevated metabolic demands in low temperatures.^[37] For example, in chestnut-crowned babblers (Pomatostomus ruficeps), group roosting during winter reduced overnight energy expenditure by 20-40%, enabling earlier breeding onset despite cold stress.^[33] These energy savings can be modeled using a simplified heat loss equation derived from Newton's law of cooling, where heat loss H is given by

H = k A (T_b - T_a)

with k as the thermal conductance coefficient, A as the exposed surface area, T_b as body temperature, and T_a as ambient temperature. In communal roosts, A decreases nonlinearly with group size due to huddling, yielding lower H per individual compared to solitary roosting; for instance, empirical data from finches show group A reductions of 25-40%, directly correlating with observed metabolic declines.^[31]

Predation Avoidance

Communal roosting provides several anti-predator mechanisms that reduce individual risk for participating animals. The dilution effect spreads predation risk across the group, lowering the probability that any single individual will be targeted.^[4] In birds, this manifests during roost assembly and departure, where larger groups decrease per capita attack rates by predators such as raptors. The many-eyes effect enhances collective vigilance, as more individuals scan for threats, allowing faster detection and response compared to solitary roosting.^[38] Mobbing responses, where roost members collectively harass approaching predators, further deter attacks and can drive them away, particularly in avian species that form tight-knit roosts.^[39] Additionally, the confusion effect arises during synchronized flight from roosts, as dense flocks disorient visually hunting predators, making it harder to single out prey.^[40] Empirical evidence supports these mechanisms across taxa. In butterflies, a 2012 field study on communal roosts of unpalatable species like Heliconius erato demonstrated that grouped individuals experienced fewer attacks from avian predators than solitary ones, attributing this to both dilution and collective aposematism that signals toxicity.^[4] This deterrence extends to eavesdropping behaviors, where predators monitor roost signals but hesitate due to the aggregated warning display. For birds, alarm call propagation within roosts amplifies threat detection; a study on mixed avian communities showed that alarm calls from one individual rapidly spread through the group, prompting evasive actions and reducing predation success.^[41] Quantitative models illustrate the efficacy of these mechanisms, particularly the many-eyes effect. The probability of detecting a predator increases with group size according to the formula:

P(\text{detect}) = 1 - (1 - p)^n

where p is the detection probability of a single individual and n is the group size; as n grows, P(\text{detect}) approaches 1, enabling earlier warnings in larger roosts.^[42] These findings underscore how communal roosting integrates multiple anti-predator strategies to minimize individual exposure.

Costs

Energy and Travel Demands

Communal roosting imposes substantial metabolic costs on participating animals, primarily through the energy required for daily commutes between foraging areas and centralized roost sites. In birds, these long-distance flights can elevate overall daily energy expenditure by 1.5% to 18.8% of the premigratory budget, with costs exceeding 10% for nearly 30% of roost sites in species like the whimbrel (Numenius phaeopus).^[43] This increase stems from the high metabolic demands of sustained flight, which often requires individuals to allocate a significant portion of their foraging time to travel rather than energy intake. Opportunity costs also arise, as time spent commuting reduces opportunities for feeding or other activities, potentially leading to lower net energy gains over the day.^[44] Evidence from tracking studies highlights how these demands manifest in specific taxa. For instance, barn swallows (Hirundo rustica) tracked at pre-migratory roosts in Europe accumulate substantial fat reserves, reaching peak levels in September–October to prepare for long-distance travel, which may compensate for the energetic toll of commuting to communal sites.^[45] Similarly, models of optimal roost distance in shorebirds balance these flight costs against benefits like safety, predicting that birds select roosts within a threshold distance where travel energy does not exceed 20–30% of daily needs, beyond which alternative solitary roosting becomes preferable.^[46] These costs are exacerbated in fragmented habitats, where dispersed foraging patches force longer commutes to remaining suitable roosts, amplifying energy demands and potentially reducing reproductive success.^[47] In urban environments, artificial lights further disrupt patterns by prolonging activity periods and altering roost arrival times; a 2025 study using acoustic monitoring found that light pollution extended birds' daily vocal activity by an average of 50 minutes in affected areas.^[48] The energy cost of such travel can be approximated using biomechanical models of flight. For horizontal commuting flights, the total energy expenditure E_{\text{travel}} includes terms for gravitational work if elevation changes occur and aerodynamic drag:

E_{\text{travel}} = m g h + \int \left( \frac{1}{2} \rho v^3 C_d A \right) dt

where m is body mass, g is gravitational acceleration, h is elevation gain, \rho is air density, v is flight speed, C_d is the drag coefficient, and A is the effective cross-sectional area. This formulation underscores how factors like body mass and distance directly scale the metabolic burden.^[49]

Competition and Interference

In communal roosts, interference competition manifests through direct aggressive interactions where dominant individuals displace subordinates to secure preferred positions, such as central or sheltered spots that offer better protection from wind and cold.^[50] For instance, in rooks (Corvus frugilegus), adults frequently push juveniles from higher, more insulated branches during inclement weather, enforcing a peck-right hierarchy based on age and sex.^[50] Similarly, long-tailed tits (Aegithalos caudatus) exhibit dominance-based contests upon roost arrival, with higher-ranking birds claiming inner positions that minimize exposure to predators and elements, while subordinates settle for peripheral sites.^[51] Exploitative competition arises as roost-mates vie for advantageous departure points that facilitate access to prime foraging areas, often leading to pre-dawn skirmishes over optimal takeoff locations. In common mynas (Acridotheres tristis), such rivalries intensify at urban roosts, where aggressive displays and chases peak during evening arrivals to claim spots overlooking resource-rich patches.^[52] Dominance hierarchies further mediate access, with established peck orders—stable across seasons in captive corvids—determining who gains thermoregulatory benefits or information cues, while lower-status birds face repeated exclusions.^[50] Evidence from field observations highlights aggression surges at roost assembly; rook studies document elevated displacement rates at dusk, correlating with flock size and resource scarcity.^[50] In mixed-species roosts, heterospecific interference can occur, though primarily documented at feeding sites; for example, black vultures (Coragyps atratus) dominate turkey vultures (Cathartes aura) during scavenging.^[53] A 2018 study on black vulture roosts found age-based hierarchies drive intraspecific conflicts, with juveniles losing the majority of encounters to adults.^[54] These competitions impose fitness costs on subordinates, including heightened energy expenditure from displacement and poorer access to roost-derived benefits like foraging leads, which ties into the social refuge-territory prospecting hypothesis where status dictates prospecting opportunities.^[55] Subordinate birds often experience reduced overwinter survival due to these exclusions.^[50] Urbanization exacerbates competition in communal roosts by clumping resources like artificial lights and waste, drawing denser flocks and intensifying interference among species.^[56] In such settings, dominant urban adapters like mynas monopolize central roost zones, marginalizing natives and altering community structure.^[56]

Disease and Parasite Transmission

Communal roosting in animals such as birds and bats increases the risk of disease and parasite transmission due to high-density aggregations that promote close physical contact and environmental contamination. Ectoparasites, including lice, mites, and bat flies, spread readily through direct body contact during roosting, while endoparasites and pathogens like viruses are transmitted via fecal-oral routes from accumulated droppings in roost sites. For instance, in bats, dense colonies facilitate the transfer of ectoparasites such as Streblidae bat flies, which detach from hosts to pupate in stable roost structures like caves, leading to higher infestation rates in permanent roosts compared to transient ones.^[57]^[58] In birds, similar mechanisms operate, with communal roosts exacerbating the spread of intestinal parasites like Isospora through fecal contamination, particularly in species such as red-billed choughs where non-breeding individuals roosting communally exhibit infection rates up to 36.2%, compared to 0% in solitary breeding pairs.^[59] Ectoparasite loads, such as lice and fleas, also peak in medium-sized bird groups (around 7 individuals) in species like speckled mousebirds, due to increased contact opportunities without sufficient social grooming to mitigate spread. Pathogens like avian influenza can propagate via respiratory secretions and feces in shared roosts, as seen in common myna aggregations where large roosts have been linked to potential dissemination of salmonellosis, Newcastle disease, and avian influenza through contaminated environments.^[60]^[61] Studies on bat roosts demonstrate elevated parasite loads correlating with colony size; for example, prevalence and intensity of ectoparasites like Spinturnix mites and Streblid flies increase significantly in larger colonies, with bats in enclosed, permanent roosts showing up to three times higher fly species diversity and mean intensity than those in open foliage. In birds, research from the 2010s and 2020s, including on choughs and mousebirds, confirms higher endoparasite and ectoparasite burdens in communal roosters, with roost structure influencing fecal contact and thus transmission efficiency. Bat colonies have been central to outbreaks of pathogens like White-nose syndrome fungus (Pseudogymnoascus destructans) and Hendra virus, where larger hibernacula accelerate spread, leading to rapid population declines.^[58]^[57]^[59] Quantitative models of transmission in communal roosts emphasize density effects; for density-dependent pathogens in bats, the basic reproduction number R_0 exceeds 1 when \beta_d N_r > v, where \beta_d is the transmission rate per density, N_r is roost size, and v is host recovery rate, resulting in faster disease escape times in large roosts (e.g., full prevalence in 25 days for 333 bats versus slower in small groups of 40). These models predict higher prevalence in centralized large roosts compared to dispersed small ones, balancing evolutionary trade-offs with benefits like thermoregulation.^[62] Animals mitigate these risks through behavioral adaptations, such as roost switching in bats to reduce ectoparasite buildup via fission-fusion dynamics, and allogrooming in birds to limit ectoparasite loads in larger groups. Infected bats may also reduce clustering to curb pathogen spread, highlighting evolutionary pressures that offset roosting costs against gains in energy conservation and predation avoidance.^[58]^[60]^[62] In insects, such as passion-vine butterflies (Heliconius spp.), communal roosts of 4–5 individuals increase vulnerability to parasite transmission, including protozoans and bacteria via close contact, though collective defenses like warning coloration may partially offset risks.^[4]

Examples by Taxon

Birds

Communal roosting is a widespread behavior among avian species, where birds aggregate in large groups at night or during rest periods, often in trees, reeds, or artificial structures. This phenomenon is particularly prevalent in corvids, passerines, and swallows, serving functions such as information sharing for foraging and reducing predation risk through dilution effects. In many cases, roosts form seasonally, with sizes varying from dozens to hundreds of thousands of individuals depending on the species and environmental conditions.^[5] Rooks (Corvus frugilegus), a corvid species native to Eurasia, exemplify large-scale communal roosting during winter, with flocks numbering in the thousands gathering in tall trees or woodlands. These roosts facilitate information transfer about foraging sites, as birds from distant areas join to share knowledge of food resources, enhancing winter survival in harsh conditions. Studies of rook roosting behavior have shown that the spatial organization within the roost, including dominance hierarchies, influences access to sheltered positions that minimize heat loss.^[63]^[64] Tree swallows (Tachycineta bicolor), a North American migrant, form communal roosts of thousands to hundreds of thousands in reed beds or marshes, particularly during the post-breeding and migration periods. This aggregation provides a predation dilution effect, where the probability of any single bird being targeted by predators like hawks decreases in larger groups. Radio-telemetry research indicates that individual swallows switch between roosts frequently, driven by a mix of conspecific attraction and anti-predator benefits.^[6]^[65] Eurasian crag martins (Ptyonoprogne rupestris), found across Europe and Asia, exhibit communal roosting under bridges, cliffs, or buildings, with flocks reaching up to 2,000 individuals in urban and semi-urban settings. These roosts, often on ledges or overhangs, support post-breeding flocking and provide shelter from weather while allowing quick access to insect foraging areas. In South Asia, migrant crag martins join mixed flocks with related species for communal roosting on building ledges during winter.^[66] Seasonal patterns are evident in many migrant birds, where communal roosts intensify during non-breeding periods to aid energy conservation and predator avoidance. For instance, Neotropical migrant songbirds like the northern waterthrush (Parkesia noveboracensis) form roosting aggregations on wintering grounds in the Neotropics, peaking in the dry season when resources are patchier. These seasonal roosts link directly to migration staging, as seen in sandhill cranes (Antigone canadensis), where over 300,000 individuals roost communally in river channels during spring migration to forage efficiently before continuing north. Similarly, chimney swifts (Chaetura pelagica) and purple martins (Progne subis) use urban structures like chimneys and bridges as staging roosts, gathering in swirling flocks of thousands before departing for South America.^[67]^[68]^[69] Urban adaptations have enabled persistent communal roosting in cities, with species like European starlings (Sturnus vulgaris) utilizing buildings and parks for large winter roosts. A 2024 study employing passive acoustic monitoring demonstrated that automated sound detection can accurately estimate roost sizes in urban environments, revealing stable aggregations of starlings despite noise pollution. Mixed-species roosts occur in some avian communities, such as those involving birds and bats sharing natural cavities or artificial sites, potentially offering mutual thermoregulatory advantages through shared warmth.^[70] Recent observations in 2025 highlight roost stability amid urban bird community shifts, as cities like those in boreal regions maintain diverse roosting sites despite increasing urbanization gradients. In green urban areas, species richness and roost fidelity remain consistent, supporting migrant populations through restored habitats and reduced habitat fragmentation. These patterns underscore the resilience of communal roosting as an adaptive strategy in human-modified landscapes.^[71]^[72]

Insects

Communal roosting in insects primarily occurs among ectothermic species with limited mobility, such as certain Lepidoptera and Hymenoptera, where aggregations serve ecological functions distinct from those in more mobile vertebrates. These behaviors often involve nocturnal resting sites or overwintering clusters that enhance survival through density-dependent mechanisms. Unlike birds or mammals, insect roosts emphasize passive thermoregulation and predation deterrence via numerical abundance rather than active defense or social coordination. In butterflies, communal roosting is well-documented in species like those in the genus Heliconius, which form nightly aggregations in vegetation to deter predators. A 2012 study demonstrated that these aposematic passion-vine butterflies benefit from group sizes that reduce predation risk, with experimental evidence showing predators avoiding larger roosts due to increased vigilance and dilution effects, though benefits plateau beyond optimal densities. Monarch butterflies (Danaus plexippus) exhibit large-scale overwintering clusters in Mexican oyamel fir trees, where high densities—estimated at 6.9 to 60.9 million individuals per hectare—facilitate microclimate stabilization against cold, indirectly supporting predation avoidance through sheer numbers.^[73] Bees, particularly honeybees (Apis mellifera), utilize hives as permanent communal roosts where clustering enables precise thermoregulation. Colonies adjust bee density through migration and vibrational heating to maintain core temperatures around 35°C, critical for brood development during cold periods. In swarming clusters, collective behaviors propagate heat from inner to outer layers, allowing regulation within 1°C of optimal levels despite external fluctuations. This contrasts with solitary roosting but underscores energy conservation in dense aggregations. Moths often form communal roosts in foliage clusters or sheltered sites, such as hollow trees, providing concealment during diurnal inactivity. Species like Idia moths aggregate in tree cavities for resting, a behavior observed as daily "slumber parties" that may reduce individual exposure to predators. Overwintering clusters in some lepidopterans, including pupal or larval stages of geometrid moths, occur in leaf litter or soil, leveraging group insulation against frost. Patterns of insect communal roosting include nocturnal aggregations in Lepidoptera, where adults seek shaded foliage at dusk for protection, and overwintering clusters that persist through seasons in temperate regions. Mixed-species roosts are noted in some lepidopterans, such as lycaenid butterflies sharing sites with pierids, potentially amplifying anti-predator benefits through diverse warning signals. Chemical signaling aids roost site choice and recruitment; for instance, pheromones in Heliconius butterflies facilitate aggregation at preferred locations with low light and humidity. These dynamics highlight insects' reliance on static or semi-permanent roosts, differing from vertebrate mobility.

Mammals

Communal roosting in mammals primarily occurs among species that exhibit high sociality and face environmental pressures necessitating group aggregation for survival, with bats and certain primates serving as prominent examples. In bats, particularly insectivorous species like the Mexican free-tailed bat (Tadarida brasiliensis), massive colonies numbering in the millions form in caves, such as Bracken Cave in Texas, where up to 20 million individuals roost seasonally from March to October, providing collective benefits like reduced predation risk through dilution effects.^[74] These aggregations leverage echolocation calls, which broadcast location information publicly, allowing non-kin bats to locate and join communal roosts, facilitating information sharing about foraging sites without direct social bonds.^[75] Fruit bats, or megabats such as the Jamaican fruit-eating bat (Artibeus jamaicensis), exhibit colonial roosting patterns in foliage, tree cavities, or caves when available, often forming dense clusters in temporary sites like leaf tents that offer concealment and thermoregulation.^[76] As predominantly nocturnal mammals, bats emerge from roosts at dusk to forage and return at dawn, with communal dawn swarming behaviors in species like the noctule bat (Nyctalus noctula) aiding in synchronized re-entry and mate guarding.^[77] Among primates, lemurs demonstrate communal roosting in arboreal sites, where groups of the brown mouse lemur (Microcebus rufus) share sleeping holes in trees during the dry season, enhancing anti-predator vigilance through huddling that confuses potential threats and provides thermal benefits.^[78] Southern bamboo lemurs (Prolemur simus) select dense foliage or ground-level sites for roosting clusters, prioritizing anti-predator adaptations like elevated positions to evade terrestrial carnivores.^[79] A notable consequence of large bat roosts is the accumulation of guano, which can lead to structural damage in natural caves or human buildings by corroding surfaces, promoting fungal growth, and exerting physical weight on fragile formations or insulation materials.^[80] In the 2020s, studies have highlighted bat roosts as hotspots for disease transmission, with dense colonies facilitating the spillover of coronaviruses; for instance, genetic analyses trace SARS-CoV-2 progenitors to bat reservoirs in southern China, where roost proximity enables viral recombination and aerosol spread.^[81] These aggregations amplify risks of zoonotic outbreaks, including respiratory pathogens that impose significant health costs on both wildlife and human populations.^[82] Recent research from 2025 underscores the adaptive use of urban human structures by bats for seasonal buffering, as buildings offer stable microclimates with consistent temperature and humidity, allowing species like the big brown bat (Eptesicus fuscus) to mitigate energetic costs during reproduction and hibernation in fragmented landscapes.^[83] This reliance on anthropogenic roosts highlights evolving human-mammal interactions, where such sites serve as refugia amid habitat loss, though they necessitate management to balance conservation and public health.^[84]

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