Illinois Natural History Survey - University of Illinois

Indiana Bats in Illinois

Bats are a greatly misunderstood, feared, and persecuted group of animals. This is unfortunate because scientists have been discovering that bats play vital roles in the ecosystems in which they occur and are beneficial to humans in several ways. For example, most North American bats are insectivorous and consume vast quantities of night-flying insects, including mosquitoes and crop pests.

Twelve species of bats occur in Illinois, four of which are listed as endangered by the Illinois Endangered Species Protection Board. Two of these species, the Indiana bat (Myotis sodalis) and the gray bat (Myotis grisescens), are also federally endangered. The Indiana bat was one of the first species listed as endangered by the U.S. Fish and Wildlife Service; the total population of this species (determined by censusing hibernating bats) had declined 28 percent from 1960 to 1975. Indiana bats are associated with the major cavernous limestone (karst) regions of the midwestern and eastern United States. During the winter, they congregate in caves and abandoned mines to hibernate in tightly-packed clusters of up to 300 bats per square foot. The main hibernation sites (hibernacula) for this species are located in southern Indiana, Kentucky, and Missouri. Indiana bats leave their hibernacula in April and the species is more widely dispersed during the summer. Until recently very little was known about the summer distribution or habitat requirements of Indiana bats. The first maternity colony (reproductively active females and their young) was not located until 1971 when its roost tree (a dead American elm [Ulmus americana]) in Indiana was bulldozed. The discovery of this tree and two trees subsequently used by the colony indicated that Indiana bats roost beneath slabs of loose, peeling bark on dead trees or certain live hickories.

Bat with radio transmitter.

Most historical records of Indiana bats from Illinois were of hibernating or migrating individuals. Prior to 1985 there were summer records of reproductively active females or juveniles from only Jackson, Pike, Union, and Wabash/Edwards counties. In 1985 James Gardner and this author, both from the Illinois Natural History Survey, and James Garner from the Illinois Department of Conservation (now Department of Natural Resources) began a statewide study of the summer distribution and habitat of the Indiana bat. Their work was made possible through the cooperation of the Illinois Department of Transportation, Shawnee National Forest, U.S. Fish and Wildlife Service, and Indiana/Gray Bat Recovery Team.

Information on Indiana bat distribution was obtained by mist netting above streams and rivers. Bats use these watercourses as flyways, foraging areas, or sources of drinking water. A pair of metal poles either 6.1 or 9.2 m tall is positioned under overhanging tree branches on opposite sides of the stream channel. Two to four very fine mist nets are stacked vertically and suspended above the stream between pulley ropes attached to the poles. Using this system, it is possible to raise the top of the uppermost net to the canopy and create a wall of net that blocks most or all of the flyway above the stream. The nets are raised at dusk and checked every 10 to 15 minutes until midnight or 1:00 a.m. Bats that become tangled in the net are carefully removed and examined to determine species, sex, reproductive condition, and weight.

Map indicates Illinois counties in which the Indiana bat has been observed from 1985 to 1994.

Mist netting was conducted at 191 sites in 71 counties during the summers of 1985 through 1994. Indiana bats were captured at 35 sites in 21 counties in the southern two-thirds of the state. Several caves also were investigated during the summer and adult male Indiana bats were found roosting in two caves and one mine, including one in an additional county. Thus, there are now summer records for 22 Illinois counties, but none farther north than Henderson and Ford. Captures of reproductively active females and/or juveniles at 24 sites indicated that maternity colonies occurred in 16 of these counties.

A tree used by an Indiana bat maternity colony in Illinois was first discovered in Pike County in 1987 after hundreds of trees at numerous sites had been checked with a bat detector (which picks up bats' ultrasonic vocalizations). The availability of miniature radio transmitters (weighing 0.75 g) made it possible to track Indiana bats that had been captured in mist nets to their daytime roosts. A transmitter was glued to the back of a bat with medical skin glue, which held for about 10 days. During the day researchers with receivers and hand-held antennas homed in on the signals from the radio-tagged bats. In this way, 36 trees used by adult females and/or juveniles and an additional 15 trees used by adult males were identified. In most cases these were dead trees and the bats roosted beneath exfoliating bark; other roost sites were beneath the bark of live trees and within cavities in dead trees. Indiana bat roost trees were located in both upland and floodplain forests, typically within 500 m of a perennial or intermittent stream.

Roosting Indiana bat.

The tree species most frequently used by maternity colonies were northern red oak (Quercus rubra), slippery elm (Ulmus rubra), cottonwood (Populus deltoides), silver maple (Acer saccharinum), and shagbark hickory (Carya ovata). The mean dbh (diameter at breast height) of roost trees used by adult females and/or juveniles was 41.7 cm. Indiana bats were found to display loyalty to their summer ranges, and some roost trees were used for more than one year (although not necessarily by the same individuals). A roost tree has a limited "lifespan," however, because eventually all the bark will slough off or the tree will fall. Therefore, to be suitable roosting habitat, a forest needs to provide a continual supply of dead trees. Site and roost loyalty underscore the importance of preserving traditional habitat for this species.

Similar studies have since been conducted in other states, including Missouri and Indiana. The critically needed information acquired through these studies is being used to produce a revised recovery plan for the Indiana bat and should help with the protection of important summer habitat for this endangered species.

Joyce Hofmann, Center for Biodiversity

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