Bat Use of Highway Bridges in South-Central Montana
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Bat Use of Highway Bridges in South-Central Montana

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      We studied use of highway structures by bats in the Billings, Montana area during 2003 and 2004. We found evidence of bat use at 78 of 130 highway structures examined during summer 2003 in Carbon, Stillwater, and Yellowstone counties; 66 structures were used apparently exclusively for night roosting, and 12 bridges were day roosts. Use of highway structures was widespread throughout the study area and among the state highway system categories (Interstate: 73.2%, Primary: 47.2%, Secondary: 57.1%, Local/State Maintained: 60.0%). Day roosts were found in all highway system categories, but relatively more were in the Local/State Maintained category; maternity colonies were in all but the Interstate category. Bat species identified at day roosts included Big Brown Bat (Eptesicus fuscus), Hoary Bat (Lasiurus cinereus), Little Brown Myotis (Myotis lucifugus), and Western Small-footed Myotis (M. ciliolabrum), none of which is a state Animal Species of Concern; use of a bridge for day-roosting by the Hoary Bat is apparently the first documented case for this species. Day roosts occupied in 2003 and 2004 included maternity colonies of Big Brown Bat (2) and Little Brown Myotis (2). Use of bridges for roosting, and intensity of use at night roosts, were generally unrelated to the landscape within 3 km (1.86 miles) of the structure. Only mean percent of forest cover was significantly greater around day roost structures, but substantial overlap among unused, night roost, and day roost categories indicated that this pattern was a trend and not the major influence on structure use by bats. All day roosts were found within 8 km (5 miles) of riparian corridors. Bats used 75.9% of concrete structures, 37.5% of steel structures, and 31.6% of wooden ones. Day roosts were found in concrete or wood structures, none in steel; three maternity colonies were in wood bridges, one in a concrete bridge. Slab bridges were the least preferred concrete spans because they provided few if any protected sites for roosting bats on the underside of the deck. Night roosting bats perched on the exposed vertical surfaces of girders of concrete and steel bridges. Day roosting sites in concrete bridges included accessible expansion joints between cast-in-place and T-beam bridge sections, the longitudinal slots on the underdeck of parallel box-beam structures, and the space between two abutting bridge lanes. Day roosts in wood bridges included the narrow space between parallel girders, and the spaces between wood supports under the deck where railing posts were anchored. Of the 12 day-roosts found in 2003, nine, including all maternity colonies, were occupied on at least one visit in 2004. Maternity colonies were occupied from early June (possibly late May) into September, but dispersal from them had begun by late August.
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