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Gray Bat Day Roosts in Concrete Barriers on Bridges during Migration.


Gray bats (Myotis grisescens) are found throughout karst areas of the southeastern United States where it primarily roosts in caves, although noncave summer roost sites have been found in concrete box culverts in Arkansas, Illinois, Kansas, Missouri, Tennessee, and Virginia (Hays and Bingman, 1964; Elder and Gunier, 1978; Timmerman and McDaniel, 1992; Johnson et al., 2002; Gerdes, 2016; Powers et al., 2016). Males and females hibernate together in large colonies in a small number of caves primarily in Alabama, Arkansas, Florida, Kentucky, Missouri, and Tennessee (Brady et al., 1982). In the spring they migrate as far as 775 km to hundreds of caves used as separate bachelor and maternity colonies (Decher and Choate, 1995). Spring migration takes place from March to late May. Fall migration back to hibernacula occurs from late August to mid-November (Tuttle, 1976; Gerdes, 2016). It is well known gray bats forage primarily over water during summer, but very little is known concerning the flight corridors used during spring and fall migratory travel (LaVal et al., 1977; Thomas and Best, 2000; Moore et al., 2017).

Gray bats were listed as endangered under the Endangered Species Act in 1976, primarily due to population declines caused bv disturbance at roost sites. However, there are recent indications that management actions have helped aid population recovery across most, but not all, of the gray bat range (Brady et al., 1982; Sasse et al., 2007; Gore et al., 2012; Hammerson et al., 2017). The Gray Bat Recovery Plan does not include migration and associated potential threats to gray bats as a reason for the decline of the species (Brady et at., 1982). However, Tuttle and Stevenson (1977) suggested stress associated with long-distance movements could be a cause of mortality, making awareness and protection of stop-over sites and temporary roost sites all that more important. It was documented that 85% of the 71 gray bats recovered by the public at noncave sites were found sick or dead, or died after capture.

The purpose of this study was to report on a novel type of roost site used during spring and fall migration and to describe seasonal patterns of use by these endangered bats.


From 2013-2018 I attempted to locate all bridges with concrete barriers in the Ozark Highlands and the Boston Mountains ecoregions in northern Arkansas. One hundred and sixty-four bridges were identified and surveyed for bat usage. Daytime visual examinations for bat presence were made in the vertical crevices between segments of concrete barriers along the sides of the road. Width of crevices other than the large (> 25.4 mm) crevices associated with bridge deck expansion joints was measured with a ruler at 154 bridges. No other parts of the bridges were surveyed as this project was part of a separate study on eastern small-footed bats (Myotis leibii) using this specific type of roost site. When bats were observed they were coaxed from the crevice with a thin metal yardstick and captured by hand. Bat species, sex, and juvenile/adult status based on degree of epiphyseal-diaphyseal fusion (Anthony, 1988) were recorded prior to application of 2.9 mm forearm bands (Porzana Ltd., United Kingdom). A subset of 24 bridges were monitored at least ihree times a year throughout the survey period as part of the separate eastern small-footed bat study. Other bridges were surveyed throughout 2014 and 2018. An unusually large number of gray bats roosting in concrete barriers was observed in the spring of 2018; therefore, survey efforts were increased during ihe fall migration period to match the change in observed use. While no attempt was made to standardize survey effort across seasons or bridges, for purposes of visualization of seasonal observations, bridges were grouped by season into spring (March 1-May 31), summer (June 1-August 15), and fall (August l(i-November 30). The stream order at bridge sites was based upon a stream order classification layer (updated October 16, 2014) produced by the Arkansas Department of Environmental Quality and downloaded from the Arkansas GIS office web site ( The chi-square test was used to test for difference in crevice width and stream order in used and unused bridges. Due to the status of gray bats as an endangered species, specific information on locations where gray bats were captured has not been included in this article. All bridges were within 180 km of Marshall, Arkansas ((35 54'N, 92 39"W).


Gray bats were observed roosting in 21/164 (12.8%) bridges in Franklin, Izard, Johnson, Lawrence, Madison, Newton, Randolph, Sharp, and Washington counties during the course of 38/1270 (3%) surveys. Seasonal survey effort was not equal; surveys were conducted primarily in the summer (544, 43%) with fewer surveys in the fall (486, 38%) and spring (240, 19%) (Table 1). Most gray bats were observed in the spring (62, 84%), with some observations in fall (10, 14%), and two (3%) juvenile males in the summer. From 2013-2017, one to five gray bats were captured during spring surveys and two to five in fall surveys, and in 2018 this rose to 38 captures in the spring and 12 in the fall (Table 2). Most (67%) gray bats were found in bridges over 3rd and 4th order streams; however, bats used bridges over all stream from lst-6th orders and there was no statistical difference in use by stream order (chi square = 5.1232, P=0.27489. Most (67%) crevices utilized by gray bats were 12.7 mm in width (range 7.9-12.7 mm), but there was no statistically significant difference (chi square = 1.0553, P = 0.58999) in use across the range of crevice widths observed (6.35-19.1 mm). Seventy-two bats (62M, 10F) were captured alive and two were found dead.

In 75% of observations, a single gray bat was detected on the bridge. However, there were surveys in which multiple gray bats or gray bats and other species used the same crevice. A bridge in Madison County had two males using the same crevice on April 11, 2017. On April 18, 2018, a male and female gray bat were found in one crevice and three males were found in a separate crevice on that same bridge. Two male gray bats were captured in the same crevice on a different bridge in Madison County on April 18, 2018. On one occasion a male gray bat was found next to a female big brown bat (Eptesicus fuscus) in a crevice. The largest grouping of gray bats was observed on a bridge in Newton County on April 27, 2018 when 12 gray bats (11M: IF) along with four big brown bats were captured; two male gray bats were roosting in the same crevice, one male gray bat was roosting with a big brown bat, and the rest roosted singly in other crevices.

A gray bat was found dead in a crevice in a bridge in Fulton County on May 1, 2014 but was discarded without necropsy. At a bridge in Carroll County on April 27, 2018 a dead gray bat and a dead tricolored bat (Perimyotis subjlavus), both severely desiccated, were found in the same crevice as two live male gray bats. The two dead bats were examined by the Southeastern Cooperative Wildlife Disease Study; they concluded death was likely due to trauma (M.F. Dalton, pers. comm.). Potential evidence of roost fidelity across both seasons and years was demonstrated by the capture of a male bat al a bridge in Carroll County on April 10, 2014 and the recapture of that bat at the same site on October 20, 2015.


While eastern small-footed bats and northern long-eared baLs (Myotis septentrionalis) have been observed roosting in bridge barrier devices, this behavior has not been documented for gray bats (MacGregor and Riser, 1998: Thomson, 2013). The mechanism by which bats navigate during migration is not well understood and it is possible rivers serve as sensory cues as well as sources for aquatic insects used as food during the journey. Gray bats have been documented night roosting under bridges and using riverine corridors during summer foraging and this behavior has also been observed in other bat species (Johnson et al, 2002; Holland, 2007; Furmankiewicz and Kucharska, 2009; Moore et al., 2017).

My study suggests that use of concrete barriers on bridges as roost sites during migration may be common. The cause of the increased number of gray bat observations in the spring of 2018 is unclear. While the number of surveys was higher during that period than in previous years, the timing of surveys was similar. Though only 12.8% of surveyed bridges were found to have been used as roost sites, this may underestimate usage rates as bridges were Only surveyed a few times each season and the entire bridge was not examined. Previous work by Cervone el al. (2016) demonstrated that other portions of bridges may be used by gray bats during migration periods by documenting single occurrences of gray bats roosting underneath an Indiana bridge during April and September. A broader view of seasonal use patterns could be obtained by increasing survey frequency and more complete examination of potential roost sites; however, more thorough surveys of the undersides of many bridges are limited bv access difficulties caused by bridge height and the presence of deep water. The significance of bridge roosts used by a few individuals may be marginal given that hibernating gray bat populations exceed three million bats (Martin, 2007) but perhaps aerial radiotelemetry could be utilized to determine what proportion of migrating bats use concrete barriers as roost sites, as well as obtaining more detailed information on specific migration routes (Holland and Wikelski, 2009; Krauel et al., 2018).

Fensome and Mathews (2016) discussed collisions with vehicles as a cause of death in bats. Roosting in bridges may plate bats at increased risk of being struck by vehicles, especially should bats exit the barrier guardrail on the side of the road while still at relatively low flight speeds. The cause of the trauma that resulted in the death of the gray and tricolored bats found during the study is not known. Vehicular collision may be a reasonable assumption, although it seems unlikely that bats of two different species would be struck by cars and then seek shelter in the same crevice. It is possible both bats were in the crevice at the time when the trauma was caused, perhaps as a result of a predator or bridge maintenance.

The width of crevices used bv gray bats in this study was in proportion to their availability; however, a practical minimum crevice width must exist that would prevent bats from using them as roost sites. The lack of preference for bridges over higher order streams is puzzling, but this could be a consequence of combining observations within and across seasons. If gray baLs are using streams as migratory pathways this may be demonstrated bv finding them at bridges over low order streams early in the season in comparison to later in the season when they may have migrated downstream to higher order streams. Differential timing of migration bv males and female gray bats may also influence examination of within-season use of bridges over different stream orders. Obtaining data from a greater geographic range with a larger sample size of bridges with different crevice widths and over a broader variety of stream orders may be necessary to further understand the importance of these characteristics in determining roost site selection.

Management recommendations can be made based on the results of the present stud)-. First, highway management agencies in states within the range of the gray bat should ensure this type of previously unreported roost sites are surveyed for this endangered species prior to maintenance, repair, or replacement. These agencies could perform these actions during the winter or summer months when gray bats would likely not be present (White-nose Syndrome Conservation and Recovery Group, 2017). Second, should bridges utilized by gray bats need replacement, concrete barriers on the new structure should have crevices between segments with 7.9-12.7 mm widths as were utilized by gray bats in this study. Although the overall importance of bridge habitats as roosts for this species during migration is unknown, because of their endangered status it is important to protect and enhance any potential habitat utilized by this species.

Acknowledgments.--I would like to thank the Southeastern Cooperative Wildlife Disease Study at the University of Georgia for performing necropsy on bats submitted for this project and Dr. Chris Middaugh, Arkansas Game and Fish Commission for comments provided on an earlier draft of this paper.


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Caption: FIG. 1.--Concrete barrier on the side of the deck of a bridge showing vertical crevices surveyed for bats as part of this project
Table 1.--Total number of survey events and surveys with gray bats
present at bridges in northern Arkansas, 2013-2018

                    2013   2014   2015     2016

Bridge Surveys       94    232     179     143
Gray bats present     y      3       6       5

                    2017   2018   Total

Bridge Surveys      102    520    1270
Gray bats present     3     19      38

Table 2.--Total number of surveys performed and gray bats observed
(#) at bridges by season in northern Arkansas, 2013-2018

         Spring    Summer      Fall       Total

2013     27 (1)    42 (1)    25 (0)      94 (2)
2014     50 (4)    119 (0)   63 (0)     232 (4)
2015     24 (2)    53 (0)   102 (4)     179 (6)
2016     42 (4)    48 (0)    53 (2)     143 (6)
2017     36 (5)    29 (0)    37 (0)     102 (5)
2018    61 (38)    253 (1)  206 (12)   520 (51)
Total   240 (54)   544 (2)  486 (18)   1270 (74)
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Title Annotation:Notes and Discussion Piece
Author:Sasse, D. Blake
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1U7AR
Date:Jul 1, 2019
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