Highway infrastructure are omnipresent features of human activity and provide productivity benefits (e.g., transport, market expansion, production and distribution, accessibility and tourism) and connect regional development. However, linear infrastructure (e.g., roads, highways) represents one of the most consequential anthropogenic impacts on natural ecosystems. Although adverse effects from highway infrastructure exist (e.g., mortalities; barrier effects; isolation, restriction of animal movement, habitat patchiness, population fragmentation), these networks can establish otherwise esoteric microhabitats and desirable resources, (e.g., remnant vegetation, foodstuff for scavengers, sodium pools) [67, CitationLaurian, C., C. Dussault, J. P. Ouellet, R. Courtois, M. Poulin, and L. Breton. 2008. Behavior of moose relative to a road network. J of Wildlife Management, 72(7), 1550-1557. 68]CitationLeblond, M., C. Dussault, J. P. Ouellet, M. Poulin, R. Courtois, and J. Fortin. 2007. Management of roadside salt pools to reduce moose-vehicle collisions. J of Wildlife Management, 71(7), 2304-2310., and road verges may constitute “long, ribbon-like habitats,” that facilitate movement and dispersal [24]CitationCoffin, A. W. 2007. From roadkill to road ecology: a review of the ecological effects of roads. J of Transport Geography, 15, 396-406.. North American chiropteran species readily exploit these anthropogenic structures, which function as alternative roosts (i.e., diurnal roosts, nocturnal roosts, maternity roosts) and stepping-stone refugia (i.e., transitory roosts) for migratory populations.
Forty-seven microchiropteran species of 20 genera and three families populate the United States [116] CitationLoeb, S. C., T. J. Rodhouse, L. E. Ellison, C. L. Lausen, J. D. Reichard, K. M. Irvine, T. E. Ingersoll, J. Coleman, W. E. Thogmartin, J. R. Sauer, C. M. Francis, M. L. Bayless, T. R. Stanley, and D. H. Johnson. 2015. A plan for the North American Bat Monitoring Program (NABat). Gen. Tech. Rep. SRS-208. Asheville, NC; U.S. Department of Agriculture Forest Service, Southern Research Station. 100 pp. . In North America, many species are categorized as endangered or threatened, have become candidates for these categories, or are considered species of concern [83] CitationO’Shea, T. J., and M. A. Bogan, eds. 2003. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp.. In the southeast, 87% of bat species carry special conservation designations (e.g., rare, sensitive, species of concern) somewhere within their range [77] CitationMenzel, J. M., M. A. Menzel, W. M. Ford, J. W. Edwards, S. R. Sheffield, J. C. Kilgo, and M. S. Bunch. 2003. The distribution of the bats of South Carolina. Southeastern Naturalist, 2(1), 121-152.. In the western US, more than 45% of chiropterans receive the highest priority status for funding, planning, and conservation actions [106]CitationWestern Bat Working Group. 2007. Regional bat species priority matrix.. A former Department of the Interior ‘Species of Concern’ list identifies Myotis ciliolabrum, M. evotis, M. thysanodes, M. velifer, M. volans, M. occultus, M. yumanensis, Corynorhinus townsendii, Lasiurus cinereus, Euderma maculatum, Tadarida brasiliensis, Nyctinomops macrotis, Eumops perotis, E. underwoodii, Leptonycteris curasoae, L. nivalis, Choeronycteris mexicana, and Macrotus californicus [3] CitationAdams, R. A. 2003. Bats of the Rocky Mountain West: natural history, ecology, and conservation. University Press of Colorado, Boulder, Colorado.. Of these 18 species, 77.7% (14) exploit bridges as either nocturnal or diurnal roosts.
As the quantity and quality of natural roost sites dwindle, manmade infrastructures (e.g., mines, buildings, culverts, bridges) function as vital habitat for bat communities worldwide. Interior spaces of these anthropogenic structures proffer physical and thermal characteristics reminiscent of natural roosts and therefore, have become amenable substitutes. The successfulness of the Congress Avenue Bridge has prompted surveys and studies of bridge thermodynamics, microclimates, roosting ecology, species occurrence and activity patterns. A comprehensive, exhaustive literature search generated numerous articles and publications documenting bridge use by bats worldwide. Keeley and Tuttle [57] CitationKeeley, B. W., and M. D. Tuttle. 1999. Bats in American bridges. Bat Conservation International, Inc., Austin, Texas, Resource Publication, 40 pp. list 24 North American species that exploit bridges and “thirteen others likely do” (See ‘Structure Inhabiting Species‘). Adams [3] CitationAdams, R. A. 2003. Bats of the Rocky Mountain West: natural history, ecology, and conservation. University Press of Colorado, Boulder, Colorado. surmises that 93 percent of California’s rare bat species benefit from these structures. Keeley and Tuttle estimate 4,250,000 bats currently inhabit 211 highway structures. Within the southern United States, estimates approach 33 million bats per 3,600 highway structures [1] CitationAmerican Association of State Highway and Transportation Officials, Center for Environmental Excellence. 2008. Bird and bat roosts in bridges. In Environmental stewardship practices, procedures, and policies for highway construction and maintenance. NCHRP Project 25-25 (04)..

Roost site selection has considerable consequences for the survival and reproduction of bats. A multitude of factors influence the roosting ecology of these mammals; including insect availability, predator avoidance, sociality, thermoregulation and roost structure, location, availability and abundance. The abundance and diversity of available roost structures influence the distributions of most temperate zone species. Menzel et al. [77]Menzel, J. M., M. A. Menzel, W. M. Ford, J. W. Edwards, S. R. Sheffield, J. C. Kilgo, and M. S. Bunch. 2003. The distribution of the bats of South Carolina. Southeastern Naturalist, 2(1), 121-152. and Bogan et al. [13]Bogan, M. A., P. M. Cryan, E. W. Valdez, L. E. Ellison, and T. J. O’Shea. 2003. Western crevice and cavity-roosting bats. In O’Shea, T. J., and M. A. Bogan, eds. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp. affirm that bat community richness closely parallels the diversity of available roost structures.
Cavity quality directly influences reproductive success; either directly via juvenile survival, or indirectly via their subsequent growth and development. Various factors influence postnatal growth rates and survivorship of young; including climate, availability and abundance of foodstuff, roost temperature, age, mother’s nutritional and hormonal condition, parasite loads, colony size, and presumably anthropogenic factors [4]Allen, L. C., C. S. Richardson, G. F. McCracken, and T. H. Kunz. 2010. Birth size and postnatal growth in cave- and bridge-roosting Brazilian free-tailed bats. Journal of Zoology 280, 8-16.. Low temperatures may delay gestation, reduce birth size, or slow postnatal growth [102]Tuttle, M. D. 1976. Population ecology of the gray bat (Myotis grisescens): factors influencing growth and survival of newly volant young. Ecology 57, 587-595.. Chalinolobus tuberculatus selects for large roost-tree diameters and thicker cavity walls that precipitate warmer, and more stable microclimatic conditions [95]Sedgeley, J. A. 2001. Quality of cavity microclimate as a factor influencing selection of maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand. J of Appl. Ecology, 38, 425-438.. Similarly, Myotis sodalis may choose roosts with more solar exposure because elevated temperatures accelerate embryonic and juvenile development [19]Carter, T. C., and G. A. Feldhamer. 2005. Roost tree use by maternity colonies of Indiana bats and northern long-eared bats in southern Illinois. Forest Ecology and Management, 219, 259-268..
The energy requirements of breeding females are extremely high during pregnancy and lactation; approximately 2.5-5 times the level of non-reproductive energy expenditure [75]McLean, J. A., and J. R. Speakman. 1999. Energy budgets of lactating and non-reproductive brown long-eared bats (Plecotus auritus) suggest females use compensation in lactation. Functional Ecology 13, 360-372.. Additionally, reproductive females minimize daily torporGlossarytorpor
mechanism to conserve energy because it can negatively impact reproductive success and mother/offspring fitness [95,Sedgeley, J. A. 2001. Quality of cavity microclimate as a factor influencing selection of maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand. J of Appl. Ecology, 38, 425-438. 58,Kerth, G., K. Weissmann, and B. Konig. 2001. Day roost selection in female Bechstein’s bats (Myotis bechsteinii): a field experiment to determine the influence of roost temperature. Oecologia, 126(1), 1-9. 107]Willis, C. K. R., and R. M. Brigham. 2007. Social thermoregulation exerts more influence than microclimate on forest roost preferences by a cavity-dwelling bat. Behav Ecol Sociobiol 62, 97-108.. Kerth et al. [58]Kerth, G., K. Weissmann, and B. Konig. 2001. Day roost selection in female Bechstein’s bats (Myotis bechsteinii): a field experiment to determine the influence of roost temperature. Oecologia, 126(1), 1-9. conclude that no single optimal roost serves the energetic needs of variable weather conditions or reproductive phases. Kurta et al. [64]Kurta, A., K. J. Williams, and R. Mies. 1996. Ecological, behavioural, and thermal observations of a peripheral population of Indiana bats (Myotis sodalis). In Barclay, R. M. R., and R. M. Brigham (eds.). Bats and Forests Symposium. BC Ministry of Forestry, Victoria, BC, Canada, pp. 102–117. and Callahan et al. [18]Callahan, E.V., R. D. Drobney, and R. L. Clawson. 1997. Selection of summer roosting sites by Indiana bats (Myotis sodalis) in Missouri. J. Mammal., 78, 818–825. Abstract only. convey that individuals that comprise maternity colonies may exploit several trees to provide the total resources (i.e., cover, correct temperature) necessary [19]Carter, T. C., and G. A. Feldhamer. 2005. Roost tree use by maternity colonies of Indiana bats and northern long-eared bats in southern Illinois. Forest Ecology and Management, 219, 259-268.. These roosts may be chosen during adverse environmental conditions (e.g., rain, wind, temperature extremes) or loss of primary roost, to reduce parasite loads, access foraging grounds, or to minimize predation.
Numerous publications document the presence of maternity colonies and crèchesGlossarycrèche nursery within bridges [25, CitationDavis, R. 1966. Homing performance and homing ability in bats. Ecol. Monographs 36(3), 201-237. 100, CitationSwift, S. M. 1997. Roosting and foraging behavior of Natterer’s bat (Myotis nattereri) close to the northern border of their distribution. J. Zool., Lond., 242, 375-384. 110,CitationZahn, A. 1999. Reproductive success, colony size and roost temperature in attic-dwelling bat Myotis myotis. J. Zool., Lond., 247, 275-280. 31, CitationErickson, G. A., E. D. Pierson, W. Rainey, and P. Brown. 2002. Hitchhikers guide to bat roosts. Bat and Bridges Technical Bulletin, California Department of Transportation, Sacramento, California. 109, CitationWolf, S. A., and W. W. Shaw. 2002. Roost selection of bridges by bats in an urban area. Arizona Game and Fish Department, Heritage Grant U98007. 232 pp. 48, CitationHendricks, P., J. Johnson, S. Lenard, and C. Currier. 2004. Use of a bridge for day roosting by the hoary bat, Lasiurus cinereus. Canadian Field-Naturalist, 119(1), 132-133. 45, CitationGeluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429. 84] CitationPapadatou, E., C. Ibanez, R. Pradel, J. Juste, and O. Gimenez. 2011. Assessing survival in a multi-population system: a case study on bat populations. Oecologia, 165, 925-933.. Of 27 New Mexico bridges known to provide roosting habitat, more than 41% have documented maternity colonies (Table 4, Map 1). Allen et al. [4]Allen, L. C., C. S. Richardson, G. F. McCracken, and T. H. Kunz. 2010. Birth size and postnatal growth in cave- and bridge-roosting Brazilian free-tailed bats. Journal of Zoology 280, 8-16. found that pups born at bridge sites were larger (i.e., body mass, forearm length) and grew faster than those born at cave sites. Therefore, bridge born pups achieved adult size more rapidly, became volant earlier, and thus, acquired a selective advantage compared to cave born pups. Similarly, Geluso et al. [44]Geluso, K. N., J. S. Altenbach, and D. E. Wilson. 1981. Organochlorine residues in young Mexican free-tailed bats from several roosts. Am. Midl. Naturalist 105(2), 249-257. describe bridge inhabiting bats exhibiting heavier weights than individuals from caves, including Carlsbad Caverns. Additionally, body condition (i.e., ratio of body mass to forearm length) was greater for females inhabiting bridges [4]Allen, L. C., C. S. Richardson, G. F. McCracken, and T. H. Kunz. 2010. Birth size and postnatal growth in cave- and bridge-roosting Brazilian free-tailed bats. Journal of Zoology 280, 8-16..
Bennett et al. [8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399. describe the affinity for anthropogenic structures increasing May-August, when maternity colonies appear. These structures are extraordinarily valuable because they provide females with warmer and more stable thermal environments that impart substantial energetic benefits. Maternity roosts are insulated against temperature extremes and have significantly smaller temperature and humidity ranges relative to external, ambient conditions (Sedgeley 2001; Smith and Stevenson, unpubl. data, Figure 1).
Females communally roost in maternity colonies, and therefore, social thermoregulation may also play an important role. Although most scientists concur that microclimate strongly influences roost site selection, Willis and Brigham [107]Willis, C. K. R., and R. M. Brigham. 2007. Social thermoregulation exerts more influence than microclimate on forest roost preferences by a cavity-dwelling bat. Behav Ecol Sociobiol 62, 97-108. propose that sociality may preeminently influence roost preferences.
Willis et al. [108]Willis, C. K. R., C. M. Voss, and R. M. Brigham. 2006. Roost selection by female, forest living big brown bats (Eptesicus fuscus). J Mammal 87, 350-354. demonstrate preferences to tree cavities with relatively large volumes, which permit individuals to roost collectively. Additionally, colony size was positively correlated with cavity volume. The microclimate within maternity roosts can transform substantially from the metabolic heat of its occupants, increasing internal temperature 5-10 °C above that of unoccupied roosts [95]Sedgeley, J. A. 2001. Quality of cavity microclimate as a factor influencing selection of maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand. J of Appl. Ecology, 38, 425-438.. Willis and Brigham [107]Willis, C. K. R., and R. M. Brigham. 2007. Social thermoregulation exerts more influence than microclimate on forest roost preferences by a cavity-dwelling bat. Behav Ecol Sociobiol 62, 97-108. illustrate a significant positive correlation between quantity of bats, maximum daily roost temperature, and energy savings. On average, one individual (normal body temperature) can conserve ca. 9% of its daily energy budget by roosting with conspecifics, which can increase to 53% in a colony of 45.
Smith and Stevenson (unpubl. data) document bat occupancy of ten distinctive structures as roosts; including concrete spalls, .” crevices in the 2 terminal spans (transverse joint between eastbound and westbound decks), steel drainage pipes, external expansion joints between deck slabs, internal expansion joints above column caps, “open” beams between centermost girders (.639 m), typical “open” beams (1.459 m), bolt cavities within pre-insulated pipes, space between piers and pedestals, and cliff swallow (Hirundo pyrrhonota) nests (Gallery 1). Structures with the greatest volume (i.e., internal and external expansion joints, and open beams) receive the most use (Smith and Stevenson, unpubl. data). Preferred old-growth redwood hollows had greater hollow volumes, greater diameters, and were closer to water than less frequently used trees [41]Gellman, S. T., and W. J. Zielinski. 1996. Use by bats of old-growth redwood hollows on the north coast of California. J of Mammalogy, 77(1), 255-265.. Similar to concrete bridges, these trees exhibit relatively stable temperatures and humidity, comparative permanence, protection from inclement weather, and spacious internal flight areas.

Roost sites adjacent to optimal foraging areas (e.g., lakes, rivers) are energetically ideal, because the foraging of insectivorous bats requires a high rate of energy expenditure. Additionally, females increase food consumption to accommodate the energy demands of reproduction. For Myotis lucifugus and M. velifer, food consumption increases approximately 45% from pregnancy to lactation, and between early and mid-lactation, T. brasiliensis increases energy intake by 82% [75]McLean, J. A., and J. R. Speakman. 1999. Energy budgets of lactating and non-reproductive brown long-eared bats (Plecotus auritus) suggest females use compensation in lactation. Functional Ecology 13, 360-372.. Flight durations (i.e., foraging times) may increase and presumably, influence site selection. Therefore, roost locales that minimize travel distances are preferable for both sexes. Evelyn et al. [32]Evelyn, M. J., D. A Stiles, and R. A. Young. 2004. Conservation of bats in suburban landscapes: roost selection by Myotis yumanensis in a residential area in California. 115, 463-473. Abstract only. document that M. yumanensis select roosts close to water and ‘distance to water’ was one characteristic that differentiated roosts from random comparisons. Similarly, a meta-analyses of 56 studies by Kalcounis- Ruppell et al. [56]Kalcounis-Ruppell, M. C., J. M. Psyllakis, and R. M. Brigham. 2005. Tree roost selection by bats: an empirical synthesis using meta-analysis. Wildlife Society Bulletin, 33(3), 1123-1132. affirms that roosts of cavity inhabiting species were closer to water than random parities. M. grisescens demonstrates a dramatic correlation between colony location and major bodies of water [102]Tuttle, M. D. 1976. Population ecology of the gray bat (Myotis grisescens): factors influencing growth and survival of newly volant young. Ecology 57, 587-595. Abstract only.. Similarly, the activity centers of female M. evotis were significantly closer to water than random points, with the probability of use decreasing with distance to available water [103]Waldien, D. L., and J. P. Hayes. 2001. Activity areas of female long-eared myotis in coniferous forests in western Oregon. Northwest Science 75, 307-314..
Growth success, percent of low weight pups, and mortality closely correlate with travel distances to foraging areas [102]Tuttle, M. D. 1976. Population ecology of the gray bat (Myotis grisescens): factors influencing growth and survival of newly volant young. Ecology 57, 587-595. Abstract only.. Differential energy expenditure relative to travel may contribute to the disparate birth sizes between cave- and bridge- born pups found by Allen et al. [4]Allen, L. C., C. S. Richardson, G. F. McCracken, and T. H. Kunz. 2010. Birth size and postnatal growth in cave- and bridge-roosting Brazilian free-tailed bats. Journal of Zoology 280, 8-16.. Because water and bridge localities often converge, bats inhabiting bridges have shorter foraging commutes than those inhabiting caves. Shortened commutes and foraging times allow pregnant females to allocate more resources and energy and nutrients to reproductive growth and milk production, respectively.

Frequent movements between diurnal shelters occur for various mammalian species (e.g., fox, Vulpes vulpes; skunk, Mephitis mephitis; badger, Meles meles; raccoon, Procyon lotor) [72, Lewis, S. E. 1995. Roost fidelity of bats: a review. J of Mammalogy, 76(2), 481-496. Abstract only. 101]Trousdale, A. W., D. C. Beckett, and S. L. Hammond. 2008. Short-term roost fidelity of Rafinesque’s big-eared bat (Corynorhinus rafinesquii) varies with habitat. J of Mammalogy, 89(2), 477-484.. In chiropteran species, where inter- and intraspecific variability exists, the phenomenon of roost switching is not well understood.
The relative availability and permanency of roosts may affect roost selectivity and subsequent fidelity. Lewis [72] Lewis, S. E. 1995. Roost fidelity of bats: a review. J of Mammalogy, 76(2), 481-496. Abstract only., and Kunz and Lumsden [61]Kunz, T. H., and L. F. Lumsden. 2003. Ecology of cavity and foliage roosting bats. In Kunz, T. H., and M. B. Fenton, eds. Ecology of Bats, pp. 3-89. The University of Chicago Press, Chicago. illustrate that fidelity corresponds to roost permanency and inversely relates to roost availability. Bats, therefore, exhibit low fidelity for ephemeral, abundant sites (e.g., dead or senescent trees, exfoliating bark) and strong fidelity to relatively rare, permanent structures (e.g., manmade bridges, buildings, caves). Utilization of impermanent roosts may be offset by location, and therefore, lower commuting costs; lower ectoparasite levels; and a relatively high abundance and availability of sites. Thus, bridges may precipitate higher roost fidelity; the benefits of which include lower costs relative to roost searching, site familiarity, and the ability to maintain social relationships [72, Lewis, S. E. 1995. Roost fidelity of bats: a review. J of Mammalogy, 76(2), 481-496. Abstract only. 21]Chung-MacCoubrey. 1999. Maternity roosts of bats at the Bosque del Apache National Wildlife Refuge: a preliminary report. In Finch, D. M., J. C. Whitney. J. F. Kelly, and S. R. Loftin. 1999. Rio Grande ecosystems: linking land, water, and people. Toward a sustainable future for the Middle Rio Grande Basin. 1998 June 2-5; Albuquerque, NM. Proc. RMRS-P-7. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 245 p..
Trousdale et al. [101]Trousdale, A. W., D. C. Beckett, and S. L. Hammond. 2008. Short-term roost fidelity of Rafinesque’s big-eared bat (Corynorhinus rafinesquii) varies with habitat. J of Mammalogy, 89(2), 477-484. and Bennett et al. [8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399. report high lability of C. rafinesquii to abundant, “not exceptionally stable” hollow trees, and site-faithfulness to comparatively permanent caves and manmade structures. Similarly, Brigham [16]Brigham, R. M. 1991. Flexibility in foraging and roosting behaviour by the big brown bat (Eptesicus fuscus). Canadian Journal of Zoology, 69(1), 117-121. affirms that E. fuscus populations were “tenaciously loyal” to anthropogenic structures and rock crevices, but noncommittal to tree cavities. Goldingay and Stevens [46]Goldingay, R. L., and J. R. Stevens. 2009. Use of artificial tree hollow by Australian birds and bats. Wildlife Research, 36, 81-97. propose that the rate of collapse of hollow-bearing trees may exceed replacement. Consequently, many native Australian, cavity-dependent species are presently listed as threatened. A study by Carter and Feldhamer [19]Carter, T. C., and G. A. Feldhamer. 2005. Roost tree use by maternity colonies of Indiana bats and northern long-eared bats in southern Illinois. Forest Ecology and Management, 219, 259-268. indicates that 25-30% of typical ephemeral roosts (i.e., tree snags with exfoliating bark) fell within one year, many within weeks; and most of the exfoliating bark pieces fell within months. Contrastingly, manmade bridges have service lives of approximately 75-100 years.
Within and between years, bats show fidelity [107]Willis, C. K. R., and R. M. Brigham. 2007. Social thermoregulation exerts more influence than microclimate on forest roost preferences by a cavity-dwelling bat. Behav Ecol Sociobiol 62, 97-108. to roost sites, patterns consistent with long-term colony stability. Bats demonstrate multiyear fidelity to night roosts, particularly bridges. Lewis [71]Lewis, S. E. 1994. Night roosting ecology of pallid bats (Antrozous pallidus) in Oregon. Am. Midl. Nat. 132, 219-226. documents Antrozous pallidus returning to night roosts within and between years. Similarly, individual Myotis volans, E. fuscus and M. yumanensis have been recaptured within the same bridge 2, 3, and 4 years, respectively, after banding [86]Pierson, E. D., W. E. Rainey, and R. M. Miller. 1996. Night roost sampling: a window on the forest bat community in northern California. In Barclay, R. M. R. and R. M. Brigham, eds. Bats and Forests Symposium, Victoria, British Columbia, Canada.. Myotis volans and M. lucifugus were recaptured 6-10 years later at the same bridges [112]Ormsbee, P. C., J. D. Kiser, and S. I. Perlmeter. 2007. Importance of night roosts to the ecology of bats. In Lacki, M. J., J. P. Hayes, and A. Kurta; editors. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland..
Worldwide, numerous studies document the propensity of bats to exploit manmade concrete and timber bridges as roost sites (Table 5-6). Certain concrete properties (e.g., thermal stability, high mean relative humidity) may confer thermoregulatory benefits similar to natural roosts. Structural precast concrete, although not an especially excellent heat conductor or insulator, exhibits thermal stability; the ability to maintain internal temperature within a certain interval, given normal external temperature oscillations. A large thermal mass provides “inertia” against temperature fluctuations; thereby, producing “chamber” temperatures beneath the bridge that are higher (often by 10 °C or more) and more stable than external ambient conditions [65, Lacki, M. J., J. P. Hayes, and A. Kurta; editors. 2007. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland. 31]Erickson, G. A., E. D. Pierson, W. Rainey, and P. Brown. 2002. Hitchhikers guide to bat roosts. Bat and Bridges Technical Bulletin, California Department of Transportation, Sacramento, California..
Timber and concrete demonstrate different behaviors, especially when subject to quick thermal variations. The distribution of temperature remains almost constant within concrete; whereas timber reacts sensitively to thermal variations [38]Fragiacomo, M., and A. Ceccotti. 2006. Long-term behavior of timber-concrete composite beams. I: finite element modeling and validation. Journal of Structural Engineering, ASCE 132, 13-22.. Similarly, the relative humidity variations of the environment affect timber and concrete differently. Several properties of timber (e.g., elasticity, shrinkage/swelling, mechano-sorption) are dependent on moisture content. Conversely, even for concrete subject to extreme conditions (e.g., long cycles of absorption within water and air drying), the relative inelastic strains are small and occur more slowly than for timber. Therefore, the influence of relative humidity on concrete is negligible [38]Fragiacomo, M., and A. Ceccotti. 2006. Long-term behavior of timber-concrete composite beams. I: finite element modeling and validation. Journal of Structural Engineering, ASCE 132, 13-22.. Thus, timber bridges may proffer numerous microclimates, with varying temperature and moisture parameters.

Sixty one percent (29) and potentially 87.2 percent (41) of North American bat species exploit manmade bridges. In order of preference; microchiropterans exploit parallel box beam bridges, cast-in-place or pre-stressed concrete girder spans [57, Keeley, B. W., and M. D. Tuttle. 1999. Bats in American bridges. Bat Conservation International, Inc., Austin, Texas, Resource Publication, 40 pp. 35]Ferguson, H., and J. M. Azerrad. 2004. Pallid bat Antrozous pallidus. Washington Department of Fish and Wildlife, Volume V: Mammals.. Of concrete cast-in-place with “chambers” on the underside of the bridge; concrete flat bottom; I-beam, parallel concrete or steel girders; and wooden; Adam and Hayes [2]Adam, M. D., and J. P. Hayes. 2000. Use of bridges as night roosts by bats in the Oregon Coast Range. J Mammal 81, 402-407. affirm that concrete cast-in-place bridges support the highest bat densities. A publication by Davis and Cockrum [26]Davis, R., and E. L. Cockrum. 1963. Bridges utilized as day-roosts by bats. Journal of Mammalogy 44, 428-430. reports of several extensive studies of bridge colonies by members of the Department of Zoology of the University of Arizona. These authors specify that, “while most bridges apparently furnish sufficient shelter at night, only those of very specific construction provide the necessary conditions to allow them to serve as day-roosts.”
Although numerous studies identify affinities to concrete structures, Smith and Stevenson (unpubl. data) and Geluso and Mink [45]Geluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429. document considerable occupancy of timber bridges. However, bats will not roost within timber structures that have newly applied creosote, an oily wood preservative with a pungent odor (Smith and Stevenson, pers. comm.,[66]Lance, R .F., B. T. Hardcastle, A. Talley, and P. L. Leberg. 2001. Day-roost selection by Rafinesque’s big-eared bats (Corynorhinus rafinesquii) in Louisiana forests. J of Mammalogy, 82(1), 166-172.). Of 36,629 day roosting bats, 99.8% inhabited timber bridges [45]Geluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429.. In these bridges; bats occupy narrow crevices (formed when two adjacent parallel beams separate), spaces above vertical supports at the culmination of spans, narrow space between beams and deck, vertical face of beams, and between guardrail post and outermost beams (Gallery 2). In Montana, a postpartum female Lasiurus cinereus with two young roosted in a narrow crevice (4 cm width, 20 cm depth) between two wooden girders 5.3 m above bare ground [48]Hendricks, P., J. Johnson, S. Lenard, and C. Currier. 2004. Use of a bridge for day roosting by the hoary bat, Lasiurus cinereus. Canadian Field-Naturalist, 119(1), 132-133.. Concurrently, a maternity colony of approximately 15 E. fuscus occupied the same crevice where the slot was narrower, ca. 1.5 m from the female L. cinereus.
Ferrara and Leberg [36]Ferrara, F. J., and P. L. Leberg. 2005. Characteristics of positions selected by day-roosting bats under bridges in Louisiana. J of Mammalogy, 86(4), 729-735. report 90.6% of Louisiana’s double-T concrete bridges had bat occupancy. Wolf and Shaw [109]Wolf, S. A., and W. W. Shaw. 2002. Roost selection of bridges by bats in an urban area. Arizona Game and Fish Department, Heritage Grant U98007. 232 pp. found 92% occupancy by eight species in the Tucson metropolis of Arizona, including 11,400 T. brasiliensis “in 1 bridge at 1 time.” In Montana, 60% (78) of structures (125 bridges, five culverts) surveyed had evidence of bat use; 66 and 12 were designated nocturnal roosts and diurnal roosts, respectively [48]Hendricks, P., J. Johnson, S. Lenard, and C. Currier. 2004. Use of a bridge for day roosting by the hoary bat, Lasiurus cinereus. Canadian Field-Naturalist, 119(1), 132-133.. Percent usage was 75.9% of concrete structures, 37.5% of steel structures, and 31.6% of wooden structures. Night roost locations were relatively exposed; “typically the vertical face of a girder (concrete or steel) near the abutment with the underside of the deck and in darker areas between girders and close to the intersection with the ground or embankment.” Day roosts were within narrow vertical spaces 3-5 cm (1.25-2”) wide and ≥ 11-20 cm (4.5-8”) in wood or concrete bridges; three maternity colonies (E. fuscus, Myotis lucifugus) roosted in wood bridges, one in a concrete bridge. All day roosts in wood bridges were underneath the deck, but five of seven in concrete bridges were in expansion joints between deck sections and near the deck edge.
Interviews with researchers from Alabama, Pennsylvania, Massachusetts, New York, Kentucky, North Carolina, Tennessee, West Virginia, Georgia, and Indiana document Myotis leibii inhabiting bridge structures of Kentucky, North Carolina, Tennessee, and West Virginia [30]Erdle, S. Y., and C. S. Hobson. 2001. Current status and conservation strategy for the eastern small-footed myotis (Myotis leibii). Natural Heritage Technical Report No. 00-19. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, VA. 34 pp.. One interviewee from North Carolina documents “the largest maternity colony in the southeast this past summer roosting in the expansion joints of a concrete bridge” [30]Erdle, S. Y., and C. S. Hobson. 2001. Current status and conservation strategy for the eastern small-footed myotis (Myotis leibii). Natural Heritage Technical Report No. 00-19. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, VA. 34 pp.. Bennett et al. [8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399. and Lance et al. [66]Lance, R .F., B. T. Hardcastle, A. Talley, and P. L. Leberg. 2001. Day-roost selection by Rafinesque’s big-eared bats (Corynorhinus rafinesquii) in Louisiana forests. J of Mammalogy, 82(1), 166-172. demonstrate a strong relationship between presence of Corynorhinus rafinesquii and construction type. C. rafinesquii demonstrates an affinity for large, concrete girder bridges and avoids flat-bottom slab bridges [8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399.. A comprehensive survey of 1,129 bridges throughout South Carolina established new records of occurrence for 10 counties, thereby determining more accurate population distributions. Mean frequency of use was 65.9% and the probability of finding bats beneath an individual bridge was 46-73%, depending on previous year occupancy. These authors, therefore, recommend an inspection interval of 3-5 times annually to determine use. Similarly, Wolf and Shaw [109]Wolf, S. A., and W. W. Shaw. 2002. Roost selection of bridges by bats in an urban area. Arizona Game and Fish Department, Heritage Grant U98007. 232 pp. “found some Tucson bridges occupied only in April, and would have thought they were never occupied had I surveyed only once in March or May.”
Ninety percent of the bat species inhabiting the Oregon Coast Range exploit bridges for roosting [2]Adam, M. D., and J. P. Hayes. 2000. Use of bridges as night roosts by bats in the Oregon Coast Range. J Mammal 81, 402-407.. Of 17 bridges surveyed in southern New Mexico, 15 (88%) contained day roosting bats and > 47% had maternity colonies with one or more species, including M. lucifugus occultus, M. yumanensis, and T. brasiliensis [45]Geluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429.. Similarly, bridges are valuable structures for maternity and nursery colonies of Myotis nattereri, a species considered vulnerable throughout Europe and the United Kingdom [100]Swift, S. M. 1997. Roosting and foraging behavior of Natterer’s bat (Myotis nattereri) close to the northern border of their distribution. J. Zool., Lond., 242, 375-384.. Numerous publications document the presence of C. rafinesquii and M. austroriparius within bridges [89, Rice, D. W. 1957. Life history and ecology of Myotis austroriparius in Florida. J of Mammalogy 38(1), 15-32. 66, Lance, R .F., B. T. Hardcastle, A. Talley, and P. L. Leberg. 2001. Day-roost selection by Rafinesque’s big-eared bats (Corynorhinus rafinesquii) in Louisiana forests. J of Mammalogy, 82(1), 166-172. 8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399.. C. rafinesquii are considered ‘Species of Concern’ throughout their range. In Mississippi, C. rafinesquii and M. austroriparius are endangered and threatened, respectively. Leptonycteris curasoae yerbabuenae, listed as endangered by USFWS, occupies bridges of the Huachuca Mountains, Arizona [51]Hollis, M. R. 1995. Formal consultation for the proposed project on State Route 90 from Interstate 10 to Huachuca City in Arizona. Arizona Department of Transportation, 1994..
A cursory survey of several structures throughout Los Lunas, Belen, and Albuquerque; information from Geluso and Mink’s [45]Geluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429. study, and a 2010- 2011 re-examination of those bridges, indicate that bat occupancy rates (88- 96%) are substantial in New Mexico. This estimate mirrors those of Louisiana and Arizona; southern states which have year-round bat activity, and relatively high species richness and diversity.
According to Kunz and Lumsden [61]Kunz, T. H., and L. F. Lumsden. 2003. Ecology of cavity and foliage roosting bats. In Kunz, T. H., and M. B. Fenton, eds. Ecology of Bats, pp. 3-89. The University of Chicago Press, Chicago., deforestation and conversion of native habitats to intensive agriculture and human development constitute the most significant threats to the density and distribution of local bat faunas. Several authors document facultative foliage roosting bats opportunistically exploiting bridge structures, including Lasiurus cinereus [48]Hendricks, P., J. Johnson, S. Lenard, and C. Currier. 2004. Use of a bridge for day roosting by the hoary bat, Lasiurus cinereus. Canadian Field-Naturalist, 119(1), 132-133., L. blossevillii [62]Kunz, T. H., and D. S. Reynolds 2003. Bat colonies in buildings. In O’Shea, T. J., and M. A. Bogan, eds. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp. and Lasionycteris noctivagans [45, Geluso, K., and J. N. Mink. 2009. Use of bridges by bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico. Southwestern Naturalist, 54(4), 421-429. Hendricks et al. 1994). As the availability and abundance of valuable roosts dwindle, occurrences of foliage roosting species exploiting anthropogenic structures may concomitantly intensify.
Sgro and Wilkins [113]Sgro, M. P., and K. T. Wilkins. 2003. Roosting behavior of the Mexican free-tailed bat (Tadarida brasiliensis) in a highway overpass. Western North American Naturalist 63, 366-373. demonstrate that bats prefer roosting over roadway segments to roosting over embankment sites (similar to Bernardo, Photo A). At the highest population densities (19 July), bats hung, oftentimes completely exposed, from protruding bolts and along the exterior bevel of the expansion joint.
Of four colonies discovered by Chung-MacCoubrey [21]Chung-MacCoubrey. 1999. Maternity roosts of bats at the Bosque del Apache National Wildlife Refuge: a preliminary report. In Finch, D. M., J. C. Whitney. J. F. Kelly, and S. R. Loftin. 1999. Rio Grande ecosystems: linking land, water, and people. Toward a sustainable future for the Middle Rio Grande Basin. 1998 June 2-5; Albuquerque, NM. Proc. RMRS-P-7. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 245 p.; the third colony, which consisted of ≥ 250 Myotis yumanensis, roosted within deep, vertical crevices (1.5 cm wide) extending the length of the underside of a small concrete bridge adjacent to the river. Erdle and Hobson [30]Erdle, S. Y., and C. S. Hobson. 2001. Current status and conservation strategy for the eastern small-footed myotis (Myotis leibii). Natural Heritage Technical Report No. 00-19. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, VA. 34 pp. document three primary habitat types of Myotis leibii; abandoned mine portals and caves, high elevation rock outcroppings, and expansion joints of concrete bridges. Rice [89]Rice, D. W. 1957. Life history and ecology of Myotis austroriparius in Florida. J of Mammalogy 38(1), 15-32. professes that during the winter, Myotis austroriparius typically roost over water within the crevices between wooden bridge timbers, storm sewers, road culverts, boat houses, and the vertical drain pipes of concrete railroad bridges.
Bats typically occupy the warmest “chambers,” the terminal spans that usually occur over land, proximal to abutments, and often within recesses that are centrally located relative to bridge width [65, Lacki, M. J., J. P. Hayes, and A. Kurta; editors. 2007. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland. 66, Lance, R .F., B. T. Hardcastle, A. Talley, and P. L. Leberg. 2001. Day-roost selection by Rafinesque’s big-eared bats (Corynorhinus rafinesquii) in Louisiana forests. J of Mammalogy, 82(1), 166-172. 7, Bennett, F. M., S. C. Loeb, and W. W. Bowerman. 2003. The distribution and contaminant exposure of Rafinesque’s big eared bats in South Carolina with an emphasis on bridge surveys. DE-AI09-00SR22188 Technical Report 03-14-R. 51 pp. 34, Feldhamer, G. A., T. C. Carter, A. T. Morzillo, and E. H. Nicholson. 2003. Use of bridges as day roosts by bats in southern Illinois. Transactions of the Illinois State Academy of Science, 96(2), 107-112. 36]Ferrara, F. J., and P. L. Leberg. 2005. Characteristics of positions selected by day-roosting bats under bridges in Louisiana. J of Mammalogy, 86(4), 729-735.. Sloping riverbanks or use of fill to stabilize bridge supports often result in end chambers being closer to the ground than center chambers, and occupied chambers occasionally are < 2 m above ground [65]Lacki, M. J., J. P. Hayes, and A. Kurta; editors. 2007. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland.. Ferrara and Leberg [36]Ferrara, F. J., and P. L. Leberg. 2005. Characteristics of positions selected by day-roosting bats under bridges in Louisiana. J of Mammalogy, 86(4), 729-735. describe roosting heights of approximately 2 m, with a tendency to roost closer to ground level than the mean height of the bridge. At the Los Lunas bridge, suitable roosts within crevices and pipes may be 2.28 m, with pipe entrances 1.20 – 2.59 m above ground (Smith and Stevenson, unpubl. data).
Dickerman et al. [27]Dickerman, R. W., K. F. Koopman, and C. Seymour. 1981. Notes on bats from the pacific lowlands of Guatemala. J of Mammalogy 62(2), 406-411. indicate that Saccopteryx bilinear, a species endemic to Central and South America that occasionally occupies bridges, roosts at heights of 1-10 m. Perlmeter [85,Perlmeter, S. I. 1996. Bats and bridges: patterns of night roost activity in the Willamette National Forest. Thesis, York University, Toronto, ON. as cited in 65]Lacki, M. J., J. P. Hayes, and A. Kurta; editors. 2007. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland. reports that concrete bridges maintain mean temperatures of 9.3-15.2 °C higher than ambient temperatures. Chambers presumably retain daytime heat, provide shelter, reduce adjacent air flow, and minimize convective heat loss. Central chambers (i.e., those over water) cool more rapidly than end chambers, which are comparatively insulated by proximity to ground and protection from air currents [85, Perlmeter, S. I. 1996. Bats and bridges: patterns of night roost activity in the Willamette National Forest. Thesis, York University, Toronto, ON. 2,Adam, M. D., and J. P. Hayes. 2000. Use of bridges as night roosts by bats in the Oregon Coast Range. J Mammal 81, 402-407. 88,Renison, N. A. 2003. Bats and bridges of the Methow Valley: night roost selection and bridge temperatures. Thesis, Western Washington University, Seattle, WA. as cited in 65]Lacki, M. J., J. P. Hayes, and A. Kurta; editors. 2007. Bats in forests: conservation and management. The Johns Hopkins University Press, Baltimore, Maryland.. Similarly, Adam and Hayes [2]Adam, M. D., and J. P. Hayes. 2000. Use of bridges as night roosts by bats in the Oregon Coast Range. J Mammal 81, 402-407. conclude that greater use of end chambers correlates to the presence of microsites with favorable thermal characteristics. Erickson et al. [31]Erickson, G. A., E. D. Pierson, W. Rainey, and P. Brown. 2002. Hitchhikers guide to bat roosts. Bat and Bridges Technical Bulletin, California Department of Transportation, Sacramento, California. propose that bridges attenuate wind and reduce wind chill and dehydration potential.

Worldwide, culverts provide additional habitat for diurnal roosting species (Table 7). Corynorhinus rafinesquii routinely exploits both bridges and culverts [66, Lance, R .F., B. T. Hardcastle, A. Talley, and P. L. Leberg. 2001. Day-roost selection by Rafinesque’s big-eared bats (Corynorhinus rafinesquii) in Louisiana forests. J of Mammalogy, 82(1), 166-172. 8]Bennett, F. M., S. C. Loeb, M. S. Bunch, and W. W. Bowerman. 2008. Use and selection of bridges as day roosts by Rafinesque’s big-eared bats. Am. Midl. Nat., 160(2), 386-399.. The African bat, Nycteris thebaica, frequently roosts within road culverts and females exhibit considerable roost fidelity, returning to the same culvert or neighboring set of culverts over several years [78]Monadjem, A. 2005. Survival and roost-site selection in the African bat Nycteris thebaica (Chiroptera : Nycteridae) in Swaziland. Belg. J. Zool., 135, 103-107.. A new subspecies of the previously monotypic Hylonycteris underwoodi was collected from culverts in Jalisco, Mexico. Additionally, specimens of Micronycteris megaliths, Sturnira ilium, Artibeus jamaicensis, and A. lituratus were obtained from the same culvert [87]Phillips, C. J., and J. K. Jones, Jr. 1971. A new subspecies of the long-nosed bat, Hylonycteris underwoodii, from Mexico. J of Mammalogy, 52(1), 77-80.. Two common neotropical species; the nectarivorous Glossophaga soricina and the frugivorous Corollia perspicillata were captured in a culvert near Albrook Air Force Base, Canal Zone, Panama [60]Klite, P. D., and M. Kourany. 1965. Isolation of salmonellae from a neotropical bat. J of Bacteriology, 90(3), 831..
Ellison et al. [29]Ellison, L. E., T. J. O’Shea, M. A. Bogan, A. L. Everette, and D. M. Schneider. 2003. Existing data on colonies of bats in the United States: summary and analysis of the U.S. Geological Survey’s Bat Population Database. In O’Shea, T. J., and M. A. Bogan, eds. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. 76 Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp. document the utilization of culverts by Myotis austroriparius, M. grisescens, M. leibii, M. septentrionalis, and M. sodalis. In Texas, Perimyotis subflavus (formerly Pipistrellus subflavus) were found overwintering in concrete box culverts along Interstate Highway 35 [91]Sandel, J. K., G. R. Benatar, K. M. Burke, C. W. Walker, T. E. Lacher, Jr., and R. L. Honeycutt. 2001. Use and selection of winter hibernacula by the eastern pipistrelle (Pipistrellus subflavus) in Texas. J of Mammalogy, 82(1), 173-178. Abstract only.. The Texas Rare Bat Survey by the Texas Parks and Wildlife Department reports the discovery of M. austroriparius wintering in culverts [22]Clark, M. K. 2003. Survey and monitoring of rare bats in bottomland hardwood forests. In O’Shea, T. J., and M. A. Bogan, eds. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp.. Fraze and Wilkins [39]Fraze, R. K., and K. T. Wilkins. 1990. Patterns of use of man-made roosts by Tadarida brasiliensis mexicana in Texas. The Southwestern Naturalist 35, 261-267. Abstract only. documented approximately 1,000 Tadarida brasiliensis on culvert walls, and evidence indicates that parturition may have occurred. These authors determine that T. brasiliensis utilizes culverts primarily in spring and summer; however, population increases in March intimate that culverts may additionally function as transitory “stop-over” roosts for migratory colonies.
Culverts proffer valuable connectivity of habitats adjacent to highways. They are readily accepted by and frequently employed by slow / low flying bat species. Bats presumably conform to the watercourse or arroyos that travel through culverts. The predominant species exploiting culverts, for both roosts and commuter routes, are those adapted to dense environments. Their wing morphology (i.e., maneuverable flight) and echolocation pulses enable these species to fly and detect small obstacles within small spaces, such as culverts. Boonman [14]Boonman, M. 2011. Factors determining the use of culverts underneath highways and railway tracks by bats in lowland areas. Lutra, 54(1), 3-16. documents cross sectional area as the most important determinant of culvert use, which was positively correlated with activity. A study by Boonman [14]Boonman, M. 2011. Factors determining the use of culverts underneath highways and railway tracks by bats in lowland areas. Lutra, 54(1), 3-16. reports that 85% of 54 culverts were used by bats.

The microclimatic properties critical to avian species parallel those of microchiropterans; and include wind, radiation, air temperature and humidity. These parameters directly influence thermoregulatory demands; therefore, nest structure and placement (e.g., orientation, shelter from inclement weather and wind, elevation, materials) become vitally important. These structures, when abandoned or unoccupied, provide ancillary roost habitat for chiropterans worldwide; including several North American species (Eptesicus fuscus, Tadarida brasiliensis, Myotis velifer, M. yumanensis) (Table 8). Shulz [93]Schulz, M. 1997. Bats in bird nests in Australia: a review. Mammal Rev., 27(2), 69-76. calculates an incidence rate of 3.9 bats per 100 Hirundo ariel nests. This author mentions the same location of the same species over two consecutive years may indicate permanent residency. A study by Schulz and Hannah [94]Schulz, M., and D. Hannah. In press. Relative abundance, diet and roost selection in the tube-nosed insect bat, Murina florium, on the Atherton Tablelands, Australia. Wildlife Research. documents Murina florid, listed as vulnerable (i.e., fewer than 50 specimens within Australia), predominately exploiting Sericornis citreogularis nests. In fact, these nest roosts accounted for 64% of total roosts found. The only documented roosts of Kerivoula papuensis, listed as rare, are those within the abandoned nests of Sericornis spp. and Gerygone mouse. The threatened species, M. florid, may inhabit the nests of Oreoscopus gutturalis at a frequency rate of 3.33% (1 in 30 nests) [93]Schulz, M. 1997. Bats in bird nests in Australia: a review. Mammal Rev., 27(2), 69-76.. The enclosed mud nests of Hirundo ariel provide roosting habitat for two rare Australian bat species, Chalinolobus dwyeri and Nyctinophilus timoriensis [53, Hyett, J. 1980. Squatters in the nests of fairy martins. Australian Bird Watcher, 8, 247–248. 54,Hyett, J., and N. Shaw. 1980. Australian mammals. A Field Guide for New South Wales, South Australia, Victoria and Tasmania. Nelson, Melbourne. 93]Schulz, M. 1997. Bats in bird nests in Australia: a review. Mammal Rev., 27(2), 69-76.. Sharma [98]Sharma, S. K. 1986. Painted bats and nests of Baya weaver bird. Journal of the Bombay Natural History Society, 81, 196. concludes that Kerivoula picta utilizes the nests of Ploceus philippinus due to the scarcity of other amenable roost sites.
Buchanan [17]Buchanan, O. M. 1958. Tadarida and myotis occupying cliff swallow nests. 39(3), 434-435. documents Tadarida brasiliensis and Myotis velifer occupying Petrochelidon pyrrhonota nests. All nests from which specimens were collected contained both Tadarida and Myotis communally, with the exception of one nest which contained only two Myotis. Buchanan reports 20 Tadarida and five Myotis within one nest, and 17 individuals from four additional nests. A note by Jackson et al. [55]Jackson, J. A., B. J. Schardien, C. D. Cooley, and B. E. Rowe. 1982. Cave myotis roosting in barn swallow nests. Southwestern Naturalist, 27(4), 463-464. describes M. velifer occupancy ratios (no. individuals to no. nests surveyed) of 11/28 (39%) and 7/29 (24%) at two concrete box culverts, respectively. These authors surmise the abundance of M. velifer in the arid southwest “may well be due to their preference for ‘crevice’ roosts and their opportunism shown here by their use of bird-built ‘crevices’ on the walls of man-made ‘caves’.” A cursory survey of one NMDOT bridge (No. 7339, Route NM 346) suggests that incidence rates are comparable to those of Jackson et al. [55]Jackson, J. A., B. J. Schardien, C. D. Cooley, and B. E. Rowe. 1982. Cave myotis roosting in barn swallow nests. Southwestern Naturalist, 27(4), 463-464. .
Only Chalinolobus moire, a species endemic to Australia, has been recorded hibernating in birds’ nests [90](Richards 1983); specifically, those of H. ariel. Interestingly, H. ariel is the only species that constructs an enclosed bottle-shaped mud nest, a structure indistinguishable from those built by Petrochelidon pyrrhonota. This convergent similarity suggests that P. pyrrhonota nests possess comparable thermal qualities and may, likewise, function as suitable winter roosts for several southwestern bat species. Although relative prevalence of this occurrence may be low, we have documented nest occupancy into December (mean temperature, 5 °C; wunderground.com).
The U.S. Migratory Bird Treaty Act (MBTA; 16 U.S.C. 703-712; Ch. 128; July 13, 1918; 40 Stat. 755) protects North American Hirundo and Petrochelidon species. NMDOT’s environmental commitments address potential impacts to these species; however, there are no commensurate regulatory policies to safeguard bats. If construction occurs during the nesting season (March-July), NMDOT will ensure that a migratory bird nest survey is conducted and unoccupied bird nests are removed. Bats that occupy H. rustica nests lay nearly prostrate within the nest cup and those within P. pyrrhonota nests (gourd-shaped enclosed structures) are typically concealed and undetectable without a borescope or fiberscope. Therefore, a survey to identify swallow activity will not discern bat occupancy.
Jackson et al.’s 1982 ‘note’ (The Southwestern Naturalist Notes Section; documents brief, notable field observations) provides not only the most contemporary, but the only estimates of occupancy rates within the United States. The distribution of Hirundo and Petrochelidon species encompasses most of North America; therefore, this occurrence may have widespread implications. We recommend either 1. an evaluation of swallow nest occupancy be a component of the statewide bridge survey, or 2. an adequate proportion of nests be inspected during the applicable phase of development (i.e., Environmental Investigations, Analysis of Alternatives; Federal Highway Administration 2009).

In temperate North America, colder months signify lower ambient temperatures and the concomitant reduction of insect prey. To circumvent this ‘energetic bottleneck,’ migratory species relocate to warmer environments, whereas other species hibernate and remain relatively inactive. Hibernating bats select ambient temperatures which allow them to drop their rate of metabolism to very low values, and maintain a temperature differential with the environment of only a few tenths of a degree Celsius (1- 2 °C above ambient), a state that has been referred to as ‘‘thermo-conformity’’ [5]Arlettaz, R., C. Ruchet, J. Aeschimann, E. Brun, M. Genoud, and P. Vogel. 2000. Physiological traits affecting the distribution and wintering strategy of the bat Tadarida teniotis. Ecology, 81(4), 1004-1014..
In the southwestern United States, certain species have been found active throughout winter. Captures by O’Farrell and Bradley [81]O’Farrell, M. J., and W. G. Bradley. 1970. Activity patterns of bats over a desert spring. J of Mammalogy, 51, 18-26. indicate that Pipistrellus hesperus, Myotis californicus, and Antrozous pallidus are active year-round in Nevada. P. hesperus and M. californicus were netted at temperatures from -8 to 33 °C. “There appears to be an alternative to hibernation or migration, at least in the warmer areas of the southwest” [81]O’Farrell, M. J., and W. G. Bradley. 1970. Activity patterns of bats over a desert spring. J of Mammalogy, 51, 18-26.. M. californicus exhibits intraspecific plasticity relative to winter activity patterns, which vary from sustained hibernation to intermittent dormancy. O’Farrell et al. [82]O’Farrell, W. G. Bradley, and G. W. Jones. 1967. Fall and winter bat activity at a desert spring in southern Nevada. Southwestern Naturalist, 12(2), 163-171. report approximately 11% of P. hesperus were netted at temperatures of 5 °C or below, indicating considerable activity at low ambient temperatures. Similarly, M. californicus were active at ambient temperatures between 2 and 6 °C. In southern and central New Mexico (Bernalillo, Eddy, Grant, Hidalgo and Socorro counties), Geluso [42]Geluso, K. 2007. Winter activity of bats over water and along flyways in New Mexico. Southwestern Naturalist, 52(4), 482- 492. captured 12 species November – March (hibernation period); with P. hesperus, M. californicus, and A. pallidus captured every month; and L. noctivagans and T. brasiliensis netted every month except January. Ambient temperatures at time of capture vary from 1- 22 °C. Also within New Mexico (Catron county), L. noctivagans and P. hesperus were captured at temperatures of -2 °C and 1 °C, respectively [114]Barbour, R. W., and W. H. Davis. 1969. Bats of America. The University Press of Kentucky, Lexington, Kentucky. 286 pp..
Geluso [43]Geluso, K. 2008. Winter activity of Brazilian free-tailed bats (Tadarida brasiliensis) at Carlsbad Cavern, New Mexico. Southwestern Naturalist, 53(2), 243-272. documents the mass exodus (tens of thousands) and return of T. brasiliensis from Carlsbad Caverns in November, February, and March, and ≥ 100 individuals in December and January. Non-migratory populations of T. brasiliensis from the southeastern United States, California, and southern Oregon are year-round residents. In these locales, this species typically roosts within buildings, forming maternity and winter colonies in warm and cooler months, respectively [62]Kunz, T. H., and D. S. Reynolds 2003. Bat colonies in buildings. In O’Shea, T. J., and M. A. Bogan, eds. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp.. Scales and Wilkins [92]Scales, J. A., and K. T. Wilkins. 2007. Seasonality and fidelity in roost use of the Mexican free-tailed bat, Tadarida brasiliensis, in an urban setting. Western North American Naturalist, 67(3), 402-408. report that, in downtown Waco, urban roosts were in use 21.4% of monitored winter days, demonstrating continued occupancy of these roosts through winter. These roosts may satisfy physiological requirements that caves do not; a tolerable thermal environment and proximity to foodstuff that remains available year-round. In California, overwintering or hibernation roosts typically occur from late fall-early spring. In many instances, these sites are also used as diurnal roosts the remainder of the year [31]Erickson, G. A., E. D. Pierson, W. Rainey, and P. Brown. 2002. Hitchhikers guide to bat roosts. Bat and Bridges Technical Bulletin, California Department of Transportation, Sacramento, California..
Hoffmeister [50]Hoffmeister, D. F. 1970. The seasonal distribution of bats in Arizona: a case for improving mammalian range maps. The Southwestern Naturalist, 15(1), 11-22. describes winter residence of M. velifer, M. thysanodes, M. californicus, M. subulatus (Myotis leibii), L. noctivagans, P. hesperus, L. cinereus, L. ega, A. pallidus, T. brasiliensis, T. molossa (Nyctinomops macrotis), M. waterhousii (Macrotus californicus), E. fuscus, P. townsendii (Corynorhinus townsendii), and E. perotis in Arizona. Menzel et al. [77]Menzel, J. M., M. A. Menzel, W. M. Ford, J. W. Edwards, S. R. Sheffield, J. C. Kilgo, and M. S. Bunch. 2003. The distribution of the bats of South Carolina. Southeastern Naturalist, 2(1), 121-152. document a citation of P. subflavus hibernacula in culverts, and houses. Benson [11,Benson, S. B. 1947. Comments on migration and hibernation in Tadarida mexicana. J. Mamm., 28, 407-408. as cited in [10]Bernardo, V. R., and E. L. Cockrum. 1962. Migration in the guano bat Tadarida brasiliensis mexicana (Saussure). J of Mammalogy 43(1), 43-64. reports eight hibernating Tadarida brasiliensis inside the crevice of a bridge, 6.5 miles southeast of Wilbur Springs, California on 1 December. Wolf and Shaw [109]Wolf, S. A., and W. W. Shaw. 2002. Roost selection of bridges by bats in an urban area. Arizona Game and Fish Department, Heritage Grant U98007. 232 pp. denote that T. brasiliensis and P. hesperus were present year-round, and document winter colonies of T. brasiliensis in Tucson bridges. Corynorhinus rafinesquii have been observed hibernating in an abandoned concrete structure in northcentral Mississippi [74]Martin, C. O., A. S. McCartney, D. Richardson, A. W. Trousdale, and M. S. Wolters. 2011. Rafinesque’s big-eared bat (Corynorhinus rafinesquii) in Mississippi: distribution, current status, and conservation needs. In Loeb, S. C., M. J. Lacki, and D. A. Miller, eds. 2011. Conservation and management of eastern big-eared bats: a symposium. Gen. Tech. Rep. SRS-145. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 157 p.. Several bridges within DeSoto National Forest were used as roosts during winter (2008), and C. rafinesquii hibernates in cisterns, wells, and culverts in the northern part of their range [74]Martin, C. O., A. S. McCartney, D. Richardson, A. W. Trousdale, and M. S. Wolters. 2011. Rafinesque’s big-eared bat (Corynorhinus rafinesquii) in Mississippi: distribution, current status, and conservation needs. In Loeb, S. C., M. J. Lacki, and D. A. Miller, eds. 2011. Conservation and management of eastern big-eared bats: a symposium. Gen. Tech. Rep. SRS-145. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 157 p.. Cel’uch and Ševčík [20]Cel’uch, M., and M. Ševčík. 2008. Road bridges as a roosts for noctules (Nyctalus noctula) and other bat species in Slovakia (Chiroptera: Vespertilionidae). Lynx (Praha), n. s., 39(1), 47-54. identify a hibernation colony of 10,000 Nyctalus noctula that occupy a road bridge in Germany.
Weaver [104]Weaver, S. P. 2012. Overwintering Brazilian free-tailed bats (Tadarida brasiliensis) in central Texas: baseline population estimates and microclimate habitat analysis. Thesis, Texas State University, San Marcos. documents T. brasiliensis occupying D’Hanis Bridge (Texas) in November, December, January and February of 2010-2011 and 2011-2012, with an estimated population of 226,350 bats. D’Hanis Bridge exhibited temperature and humidity values similar to ambient; minimum and maximum temperatures were 7.45 and 19.31 °C from December – January. This author notes 3 additional bridges in Hays County (2) and Travis County (1), Texas with overwintering populations of T. brasiliensis, indicating the northward expansion of this species’ winter range. Similarly, we documented T. brasiliensis occupancy year-round in the Bernardo bridge in 2012 and 2013. Temperatures within bat roosts of the Los Lunas bridge (ca. 33 miles north) were monitored from May 2012 to March 2013 (Smith and Stevenson, unpubl. data). December temperatures vary from -3 to 13.9 °C, mean of 4.6 °C; whereas January temperatures vary from -8.2 to 12.2 °C, mean of 2.4 °C. These parallel those temperatures at which Eptesicus fuscus (-10 to 20 °C) and Myotis spp. a (-9 to 20) hibernate [105]Webb, P. I., J. R. Speakman, and P. A. Racey. 1996. How hot is a hibernaculum? A review of the temperatures at which bats hibernate. Can. J. Zool., 74, 761-765.; however, many species require thermally stable hibernacula with < 10% oscillation. Additionally, temperatures below 0 °C may exceed the temperature threshold for most southwestern chiropterans (5 – 20 °C, 105Webb, P. I., J. R. Speakman, and P. A. Racey. 1996. How hot is a hibernaculum? A review of the temperatures at which bats hibernate. Can. J. Zool., 74, 761-765.). Bats were absent from the Los Lunas bridge between late October and mid-March; therefore, temperatures may not equally characterize the Bernardo bridge ‘winter roost’. Webb et al. document bats hibernating at ambient temperatures between -10 and 21 °C, which correspond to New Mexico’s ambient temperatures in December (-11.39 to 18.67) and January (-15.39 and 17). Weaver’s [104]Weaver, S. P. 2012. Overwintering Brazilian free-tailed bats (Tadarida brasiliensis) in central Texas: baseline population estimates and microclimate habitat analysis. Thesis, Texas State University, San Marcos. microclimatic values indicate T. brasiliensis bats are selecting colder, less stable environments during winter in central Texas.
Hibernation is characterized by torpor bout duration, lower body temperature (may fall to 2 °C), and metabolic suppression. Arousals and the return to euthermy (normal body temperatures) are energetically expensive. For Myotis lucifugus, each arousal of several hours duration costs 108 mg of fat, the equivalent of 67 days of torpor [99]Speakman, J. R., and D. W. Thomas. 2003. Physiological ecology and energetics of bats. In Kunz, T. H., and M. B. Fenton, eds. Bat ecology. The University of Chicago Press, Chicago.. At a natural rhythm of approximately one arousal every 12-15 days, arousals constitute 85% of an individual’s fat depletion through the winter. Therefore, additional arousals due to disturbance (e.g., human activity, bridge construction and maintenance) may reduce their energy supply to the point where survival of the individual is not possible.

Globally, bat populations are declining. Habitat loss and modification, climatic change, roost availability and disturbance, pesticides and pollution, disease, and human development (e.g., wind turbine facilities, urbanization) cumulatively contribute to population level impacts. Despite their decline, the increase of public awareness and conservation concern, and the occurrence of North America’s most devastating wildlife threat, white-nose syndrome; bats remain among the most neglected and misunderstood animals.

Chiropterans are intrinsic to healthy ecosystems, community integrity, and vital ecological processes. They provide valuable ecosystem services; including arthropod suppression, seed dispersal, pollination [63]Kunz, T. H., E. Braun de Torrez, D. Bauer, T. Lobova, and T. H. Fleming. 2011. Ecosystem services provided by bats. Ann. NY. Acad. Sci., 1223, 1-38. and are often considerable contributors to a country’s mammalian diversity. Boyles et al. [15]Boyles, J. G., P. M. Cryan, G. F. McCracken, and T. H. Kunz. 2011. Economic importance of bats in agriculture. Science, 332(6025), 41-42. estimate the loss of North American bats to the agricultural industry at approximately $3.7 – $53 billion annually.
Worldwide, roost availability and abundance are critical elements limiting chiropteran populations. North American bats are secondary cavity nesters; they rely on preexisting crevices and cavities. Processes of cavity creation (e.g., wood decay, fire, insect activity, excavation) and loss (e.g., deterioration, tree fall) determine the availability and abundance of quality roosts [95]Sedgeley, J. A. 2001. Quality of cavity microclimate as a factor influencing selection of maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand. J of Appl. Ecology, 38, 425-438.. In undisturbed forests, these processes achieve an approximate equilibrium; in urban environments, these ecosystem dynamics are absent. Consequently, synanthropic species are further limited by roost availability and abundance than rural or forest inhabiting species.
Additionally, bats may occupy specific roosts that provide critical microclimates for different aspects (e.g., night roost, maternity roost, transient roost, hibernaculum). Therefore, only a small subset of available roosts may be suitable and large segments of regional populations may be restricted to few specific roosts during critical times of the year [83]O’Shea, T. J., and M. A. Bogan, eds. 2003. Monitoring trends in bat populations of the United States and territories: problems and prospects. U.S. Geological Survey, Biological Resources Discipline, Information and Technology Report, USGS/BRD/ITR-2003-0003, 274 pp.. Bats spend over half their lives within their roost environment, and possess life history traits (e.g., longevity, low reproductive rates) that influence their ability to overcome population declines. Thus, quality roost sites are paramount to the survival and reproduction of an individual, and fundamentally the species.
It is evident that manmade bridges function as important components of the roosting ecology and habitat of North American bats. The identification and protection of these key roosting sites can be monumental relative to the longterm viability and conservation of endemic bat populations.
North America’s transportation system comprises 582, 976 bridges longer than 6 m (20 ft); of these, 83% traverse waterways. The U.S. Federal Highway Administration estimates an additional 12.5 million smaller structures (e.g., box culverts, drainage structures) that correlate to approximately one structure per quarter mile (400 m). An increasing proportion of national infrastructure currently exceeds or approaches its terminal service life [28]Doyle, M. W., E. H. Stanley, D. G. Havlick, M. J. Kaiser, G. Steinbach, W. L. Graf, G. E. Galloway, and J. A. Riggsbee. 2008. Aging infrastructure and ecosystem restoration. Science, 319, 286-287.; approximately 30% of bridges have classifications of “deficient” (16% structurally deficient, 13.6% functionally obsolete) [37]Forman, R. T., D. Sperling, J. A. Bissonette, A. P. Clevenger, C. D. Cutshall, V. H. Dale, L. Fahrig, R. France, C. R. Goldman, K. Heanue, J. A. Jones, F. J. Swanson, T. Turrentine, and T. C. Winter. 2003. Road ecology: science and solutions. Island Press, Washington, DC..
Construction of deficient or new structures offers an exceptional opportunity to communicate and mitigate important biotic effects relative to bat populations (Box 1).
Numerous states have become environmental stewards, actively engineering and/or retrofitting bridges to accommodate bat colonies. In the Pacific Northwest, concern relative to the importance of bridges and the sensitive status of several regional bat species were the impetus for the protection of abandoned wooden bridges on federal lands (United States Department of Agriculture Forest Service and United States Department of the Interior Bureau of Land Management 1994). Similarly, the diminution of available roost sites and the concurrent decline of North American bat populations was the incitement for the Oregon Department of Transportation’s (ODOT) proactive Bat Habitat Enhancement environmental performance standards (EPS). The ODOT’s standard outlines objectives and guidelines, “to maintain, replace, or improve roosting on bridges over waterways” [6]Barbaccia, T. G. 2011. The bridge as bat cave. Better Roads Magazine.. The Indiana DOT has developed a ‘Habitat Conservation Plan’ for the endangered Myotis sodalis as part of the improvement of transportation facilities around Indianapolis International Airport. The 2012 Indiana Bat Programmatic Biological Assessment and Programmatic Conservation Memorandum of Agreement between the Kentucky Transportation Cabinet, the Federal Highway Administration, and the U.S. Fish and Wildlife Service provides a standardized approach for biological assessments and contributes to the USFWS’s statewide conservation efforts for the Indiana bat [33]Federal Highway Administration. 2013. Programmatic agreement streamlines Indiana bat conservation efforts. Successes in Stewardship Newsletter. U.S. Department of Transportation, Federal Highway Administration.. The Texas, Florida, Georgia, Indiana, Arizona, and California Departments of Transportation are correspondingly developing canonical procedures to determine occupancy, minimize disturbance, and preserve existing or potential roosting opportunities during the management, maintenance, and demolition of bridges.

The concept of ‘road ecology’Glossaryroad ecology the ecosystem-level effect of roads and traffic has become increasingly popular, and mitigation measures (e.g., wildlife crossing structures; amphibian tunnels, culverts, overpasses, underpasses; EISs) have become important initiatives to increase permeability and habitat connectivity for wildlife inhabiting transportation corridors [37]Forman, R. T., D. S. Friedman, D. Fitzhenry, J. D. Martin, A. S. Chen, and L. E. Alexander. 1997. Ecological effects of roads: toward three summary indices and an overview for North America. Proceedings of the international conference on ‘Habitat fragmentation, infrastructure and the role of ecological engineering’ Maastricht and the Hague. 1995, 40-54.. Throughout Europe, Asia, Australia, and North America; hundreds of mitigation structures, or “ecoducts” exist; and concomitantly, numerous documents illustrating their success. However, recommendations and subsequent development of wildlife passageways target cervids, large carnivores, amphibians, and rodents [LinkTooltip Content 52,Huijser, M. P., P. McGowen, J. Fuller, A. Hardy, A. Kociolek, A. P. Clevenger, D. Smith, and R. Ament. 2007. Wildlife– 79 vehicle collision reduction study. Report to Congress. U.S. Department of Transportation, Federal Highway Administration, Washington D.C., USA. 204 pp. 23]Clevenger, A. P., and M. P. Huijser. 2011. Wildlife crossing structure handbook: design and evaluation in North America. Federal Highway Administration, Washington, DC, USA. 223 p.. Few recommendations relative to enhancing connectivity for wildlife include bats and fewer still address the effect of infrastructural development on these mammals.
Roads may affect bats via;

• vehicle collisions,
• destruction or degradation of roosts and foraging areas, and
• severance of critical commuter and migratory routes [12,Berthinussen, A., and J. Altringham. 2012. The effect of a major road on bat activity and diversity. J of Applied Ecology, 49, 82-89. 96]Semrl, M., P. Presetnik, and T. Gregorc. 2012. First proper “after construction” monitoring in Slovenia immediately reveals bats (Chiroptera) as highway traffic casualties. In Safeguarding Ecological Functions Across Transport Infrastructure. Infra Eco Network Europe International Conference. Potsdam, Germany. p. 217..

Bats employ a network of roosts, flight paths, and foraging areas within the landscape. These routes parallel linear landscape elements including treelines, hedgerows, minor roads, and watercourses (Graphic 1). The home ranges of temperate insectivorous bat species typically extend 0.5 – 5 km from their roost, with most species exhibiting high fidelity to roosts, foraging sites, and the commuting flyways that connect them.
The probability of vehicle collisions directly relates to landscape structure, and secondarily, to road features [76]Medinas, D., J. T. Marques, and A. Mira. 2012. Assessing road effects on bats: the role of landscape, road features, and bat activity on road-kills. Ecol Res, DOI 10.1007/s11284-012-1009-6. and bat ecology predictors. The highest incidence of road casualties occur where roads cross bat flyways (travel corridors), especially at junctions with forest edges and tree alleys [69]Lesiński, G. 2007. Bat road casualties and factors determining their number. Mammalia, 71(3), 138-142.. Medinas et al. [76]Medinas, D., J. T. Marques, and A. Mira. 2012. Assessing road effects on bats: the role of landscape, road features, and bat activity on road-kills. Ecol Res, DOI 10.1007/s11284-012-1009-6. indicate that high quality habitats, roost proximity, activity level, and traffic volume are primary elements of road casualties. The vulnerability of specific species correlates to flight (i.e., altitude, wing morphology, maneuverability) and foraging ecology (i.e., aerial insectivore versus gleaner, flying time, number of foraging bouts). Species with low flight altitudes that forage close to vegetation or ground (< 10 m), and that orient via guiding landscape structures are most susceptible to vehicle collisions. Mortalities may be high because most species cross at heights that place them in the paths of vehicles. Where tree canopy is high (> 20 m) and within 10 m of the highway, bats cross above traffic; where the canopy is low (< 6 m), bats cross lower and closer to traffic and therefore, probability of vehicle collisions increase [115]Russell, A. L., C. M. Butchkoski, L. Saidak, and G. F. McCracken. 2009. Road-killed bats, highway design, and the commuting ecology of bats. Endang Species Res, 8, 49-60..
A study by Zurcher et al. [111]Zurcher, A. A., D. W. Sparks, and V. J. Bennett. 2010. Why the bat did not cross the road? Acta Chiropterologica, 12(2), 337-340. demonstrates that Myotis sodalis alters direction and exhibits antipredator avoidance behavior when vehicles are present. Leziński [69]Lesiński, G. 2007. Bat road casualties and factors determining their number. Mammalia, 71(3), 138-142. determines that juveniles were killed significantly more often than adults, with increases in mortality in late summer (i.e., intense dispersal of young bats). Gaisler et al. [40]Gaisler, J., Z. Řehák, and T. Bartonička. 2009. Bat casualties by road traffic (Brno-Vienna). Acta Theriologica, 54(2), 147- 155. document similar fatality patterns, with increases in July coincident with the occurrence of volant young; and late September – early October, which may correlate to mating and autumnal migration. Leziński et al. [70]Lesiński, G., A. Sikora, and A. Olszewski. 2011. Bat casualties on a road crossing a mosaic landscape. Eur J Wildl Res, 57, 217-223. denote that more than half of road casualties occurred in July and August. Medinas et al. [76]Medinas, D., J. T. Marques, and A. Mira. 2012. Assessing road effects on bats: the role of landscape, road features, and bat activity on road-kills. Ecol Res, DOI 10.1007/s11284-012-1009-6. report that female mortality was particularly high in summer; this period encompasses births and lactation, a critical phase in the chiropteran life cycle. This may impact population viability, because newborns are dependent on mothers for survival. Kerth and Melber [59]Kerth, G., and M. Melber. 2009. Species-specific barrier effects of a motorway on the habitat use of two threatened forest living bat species. Biological Conservation, 142(2), 270-279. attribute lower reproductive success and smaller foraging areas of female Myotis bechsteinii to a major roadway that restricted habitat accessibility. In bats, both reduced reproductive success and increased mortality will profoundly affect local colony size and overall population size.

Medinas et al. affirm “the number of killed bats reported in our study is at least of the same order of the magnitude of the one often reported for wind farms,” and “bat road-kills should be also an issue of high concern for long-term bat population viability.” These authors emphasize the importance of further studies to clarify the role of bridges in road fatalities, since they may be important structures enhancing or reducing the probability of bat-vehicle casualties.
Keeley [79]National Roads Authority. 2005. Guidelines for the treatment of bats during the construction of national road schemes. Environmental Series on Construction Impacts, Dublin, Ireland, 8 pp. asserts, “the most significant impact of road construction upon bats is in the clearance phase of the scheme, namely tree-felling, the removal of hedgerows and other vegetation…” These landscape features are important habitat components; commuting routes, essential sources of insect prey, and potential roosts for both crevice- and foliage-roosting species. Additionally, roads may bisect natural flyways, and therefore, perform as barriers or filters to movement, restricting bats from accessing critical resources (e.g., foraging or roosting sites). This has the potential to influence the abundance and distribution of individuals and populations, both locally and regionally [9]Bennett, V. J., and A. A. Zurcher. 2013. When corridors collide: road-related disturbance in commuting bats. J of Wildlife Management, 77(1), 93-101.. These authors demonstrate that gaps in tree lines, hedgerows, and tree canopies can render commuting routes unsuitable for bats. To improve landscape permeability and connectivity, Bennett and Zurcher [9]Bennett, V. J., and A. A. Zurcher. 2013. When corridors collide: road-related disturbance in commuting bats. J of Wildlife Management, 77(1), 93-101. recommend the restoration (e.g., replanting shrubs and trees in gaps), enhancement (encourage interlinking tree canopies through pruning, trimming, and coppicing), and establishment of linear features; including tree lines, hedgerows, and fence lines. Lüttmann [73]Lüttmann, J. 2012. Are barrier fences effective mitigating measures to reduce road traffic bat mortality and movement barrier effects? In Safeguarding Ecological Functions Across Transport Infrastructure. Infra Eco Network Europe International Conference. Potsdam, Germany. p. 108. further emphasizes that the incorporation of fence lines and canopy cover at safe altitudes (> 4 m) provide guiding elements and barriers that prevent low flying species from entering traffic.
To minimize collisions and mortalities, alternative roosting structures should be positioned away from roadways. Traditional wooden bat boxes and “condos” do not replicate the thermal dynamics of concrete infrastructures, and novel concrete forms (RD Wildlife Management; Los Lunas, New Mexico) are not yet proven. Therefore, we propose installation of prototype concrete boxes to determine occupancy and successfulness. We recommend both 1. the installation of roosts attached to bridges, and 2. the establishment of free-standing structures within right-of-ways adjacent to these bridges, particularly where bridges span roadways. By affixing temperature and humidity recorders (LogTag Recorders Ltd., New Zealand), we can compare internal microclimates to those of actual concrete bridge roosts (Los Lunas bridge study site; Smith and Stevenson, unpubl. data).
The consideration of landscape elements (i.e., flight paths, foraging areas and roosts) prior to construction and/or demolition, and the inclusion of appropriate mitigation measures, will reduce the severity of significant impacts on bat populations.
Artificial habitats; including mines, railroad tunnels, and bridges are oftentimes closed, destroyed, or altered without first surveying. Anthropogenic activities (restoration, reinforcement or demolition of structures; landscape modification) that modify habitat parameters and thermal conditions (i.e., temperature, humidity) of hibernacula and roost sites, may cause mortality or site abandonment [30]Erdle, S. Y., and C. S. Hobson. 2001. Current status and conservation strategy for the eastern small-footed myotis (Myotis leibii). Natural Heritage Technical Report No. 00-19. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, VA. 34 pp..
The magnitude of impact varies considerably relative to the period at which these activities are conducted. The periods of pre-volancy (i.e., young are being reared and are unable to fly) and hibernation are particularly sensitive as individuals are powerless to escape. Similarly, the elimination or diminution of natural environments (e.g., forests, wetlands, hedged farmland, uncultivated land in the right of way) to construct or facilitate these activities can effectively destroy foraging areas and/or create open expanses that constitute physical barriers preventing the movement of bats [97]Sétra, CETE de l’Est, and CETE Normandie-Centre. 2009. Bats and road transport infrastructure - threats and preservation measures. Sétra Information Notes, Economics Environment Design Serie n° 91, 22 pp..
We became cognizant of the impacts of bridge maintenance activities (i.e., expansion joint replacement, vegetation removal, resurfacing) and the deficiencies of current survey techniques. We have found bats exploiting structures not previously identified (i.e., concrete spalls, insulation bolts, space between timber beams and deck, behind insulation board). Therefore, surveys to determine occupancy prior to demolitions may fail to notice bats utilizing uncommon structures and locations; and thus, timing of demolitions, bat exclusion, and maintenance activities are critical considerations during planning.
To minimize negative impacts on bat populations, a comprehensive, statewide bridge survey should be conducted with primary emphasis on those slated for construction or replacement. A survey of New Mexico’s wooden and concrete transport infrastructures would provide guidance for the strategic planning and prioritization of projects and the most appropriate techniques relative to bat species management, opportunities and/or exclusion. Any individual conducting bat surveys should possess a thorough understanding of life history characteristics, various species affected and their ecological requirements. Occupancy can oftentimes be difficult to detect without external signs (e.g., guano, urine staining), and therefore, special efforts to confirm presence / absence may be required.
In Ireland, all bat species receive legal protection via the Wildlife Act, 1976; the Wildlife (Amendment) Act, 2000; and Annex IV of the Habitats Directive. Similarly, all species are strictly protected in the United Kingdom via the Wildlife and Countryside Act, 1981; Wildlife Order, 1985; and the Habitats and Species Directive 92/43/EEC. Therefore, the documents ‘Best Practice Guidelines for the Conservation of Bats in the Planning of National Road Schemes,’ ‘Guidelines for the Treatment of Bats During the Construction of National Road Schemes,’ [79, National Roads Authority. 2005. Guidelines for the treatment of bats during the construction of national road schemes. Environmental Series on Construction Impacts, Dublin, Ireland, 8 pp. 80]National Roads Authority. 2006. Best practice guidelines for the conservation of bats in the planning of national road schemes, 48 pp. and ‘Design Manual for Roads and Bridges’ [49]Highways Agency, Scottish Executive, National Assembly for Wales Cynulliad Cenedlaethol CYMRU, and Department for Regional Development Northern Ireland. 1999. Design manual for roads and bridges (DMRB): nature conservation advice in relation to bats. Volume 10, Section 4. 40 pp. provide the most comprehensive, exemplar recommendations relative to ‘bats in bridges’.
At present, we advocate commencing construction when bats are least vulnerable (i.e., after juveniles have dispersed and individuals have not yet commenced hibernation) – prior to parturition (April – May) or following late-summer dispersal (September – October). Current recommendations propose maintenance conducted between mid- October and early April will minimize disturbance. However, we document bat occupancy through consecutive winters and therefore; conclude that bats are overwintering, with bridges functioning as hibernacula (Bernardo bridge). We believe this to be the first documentation of a bridge hibernaculum within central New Mexico. The unpredictability of this occurrence necessitates surveys prior to any works to ensure the absence of bats from bridges October through March.