Bats in Your Woods
By Joy O’Keefe
Most people associate bats with caves and it is true that caves are important as refuges where bats hibernate during winter. However, in eastern North America most bat species, including the federally endangered Indiana bat (Myotis sodalis), head to the woods when they leave caves in spring.
Forests are important to bats
Many species of bats in the Midwest roost in tall, large diameter trees during the day. Some species roost in foliage, while others roost under bark or in crevices in live or dead trees. Species like the big brown bat (Eptesicus fuscus) are likely adapted to using hollows in very large trees, but these types of trees are no longer common on the landscape. Therefore, this species has shifted to using manmade structures like barns and attics.
In general, Midwestern bat species forage for insects in open areas and along edges created by roads, streams, or edges between different habitat types. The specific areas used depend on the bat’s size and shape, and the types of echolocation calls it uses. Smaller bats are better able to maneuver in areas “cluttered” with vegetation and tend to use small openings like canopy gaps in dense forests. Larger species tend to forage in large openings like fields and clearcuts, along larger streams or roads, and above the tree canopy.
The Indiana bat is one of several bats that use forests in the Midwest during summer. Indiana bats adapted to using trees in summer at a time when forests were more common on the landscape. Pre-settlement, forests covered about 85% of Indiana’s acreage; now only 20% of Indiana is forested. Most of the remaining forest land is owned by private woodland owners. Therefore, it is important for private landowners to understand how Indiana bats use forests and how forest management might affect this species.
Indiana bats and forests
From mid-Spring to mid-Fall, Indiana bats form maternity colonies of up to 200 adult females in forested areas across much of the eastern U.S. In the Midwest, female Indiana bats and their pups usually roost in large (18 inches in diameter and larger) dead trees. Hardwoods like cottonwoods, hickories, oaks, ashes and elms are often favored, but the species of tree is less important than its characteristics. Trees with peeling or sloughing bark that receive a lot of sunlight are used most often. However, female Indiana bats sometimes roost in crevices in damaged or dead trees, under peeling bark on live trees such as shagbark hickories, or in manmade structures like bat boxes. Adult males tend to roost alone and use a much broader array of tree types and sizes, with the smallest known roost being less than 3 inches in diameter.
Indiana bats mate in the fall at their hibernation sites and females store sperm overwinter. After arriving on their summer range, females become pregnant in late April or early May. Females select a patch of woods with at least one large suitable dead tree or snag and, typically, a suite of other snags in various sizes and stages of decay. On a day-to-day basis, individuals move amongst different snags and, across a particular season, the entire colony may shift from one roost tree to another. These “switches” likely relate to the warmth of the roosts, as this can affect bats’ energy requirements. However, bats may also switch roosts to learn about alternate roosts, reduce parasite loads, move closer to foraging sites, or simply to socialize with other bats. Populations and individuals are remarkably faithful to maternity sites; for example, a colony has used the same tract of woods near the Indianapolis airport for at least 20 years.
On the landscape scale, Indiana bats often roost in bottomlands or riparian areas near streams. In the Midwest, riparian areas may be favored because they contain larger trees or it may be that bats prefer roost sites near foraging sites over streams. Adult female Indiana bats seem to prefer roosting in snags in small openings or along edges, but surrounded by live trees. Researchers think females select trees in openings to optimize the amount of sunlight on the tree which helps warm the growing pups in the roost. Because openings and edges make good foraging sites, female Indiana bats may also roost near these features to minimize the distance they need to travel each night. While females tend to roost in mature forests, male Indiana bats sometimes roost in open stands such as recently harvested areas even when potential roosts in mature forests are readily available.
In summer, a tiny (0.3 oz) pregnant or lactating female Indiana bat may spend almost the entire night hunting for insect prey, traveling up to several miles away from the maternity roost. Indiana bats primarily feed on moths, flies, beetles, and caddisflies, and may opportunistically eat a few other insect types that are occasionally abundant (e.g., winged ants). At a landscape scale, Indiana bats tend to focus on forested areas while foraging, but at a smaller scale they use open fields, riparian buffers, and other edges, in addition to foraging in the interior of mature forests. In areas where forests are fragmented by many agricultural fields, Indiana bats move or “commute” along hedgerows and stream buffers to access foraging sites in forest patches. Indiana bats may use these edges as a way to avoid crossing large open areas, or it may be that prefer to fly along forest edges because they harbor more insects. Forested riparian corridors are ideal habitats for night-flying Indiana bats, as these will generally have the low clutter characteristics of a mature forest, provide an edge for commuting or foraging, and yield a diverse prey base as insects emerge from or are drawn to the water’s surface.
As with roosts, Indiana bats tend to be faithful to foraging areas. Individual bats forage in the same specific areas night after night or year after year, and colonies tend to forage in the same overall areas within and between years.
How might timber management impact Indiana bats
Because Indiana bats rely on large dead trees as maternity roost sites, timber harvests have great potential for either positive or negative effects roosting habitat for this species. If harvests take place while bats are using trees as roosts, then bats may be evicted, injured, or killed as a result of tree removal. While there have been no planned experiments measuring the response of Indiana bats to timber harvest during the maternity season, in at least 3 known cases, occupied roost trees have been unwittingly felled during the summer and adult and juvenile Indiana bats have been killed or injured as a result. In addition, a maternity colony of Indiana bats in Missouri abandoned one of its primary roost trees when a bulldozer was used to clear away some nearby brush. In North Carolina, a population of northern long-eared bats (Myotis septentrionalis, which is a closely related species) continued to use an active timber harvest area when small buffers (~30 feet) of live trees were left surrounding their roosts. However, we do not know if Indiana bats will behave in the same manner as northern long-eared bats during an active harvest.
During large-scale timber removal, existing snags may be damaged such that roosting potential is diminished (but snags may also be created, as described below). In addition, snags may shift quickly to being unsuitable following harvest; snags left standing away from the protection of live trees are more susceptible to losing bark or snapping off in high winds and may be less suitable as roosts if they are too exposed to sun, wind, and rain. During timber harvests, retaining patches of snags buffered by live trees may help to maintain Indiana bat populations over the long-term.
Areas that have a wide variety of forest types and age classes will provide Indiana bats and other bat species with suitable foraging and roosting habitat. Ideally, harvested areas should still hold dense patches of large trees such as cottonwoods, oaks, and hickories that have the potential to become suitable maternity roost trees/snags in the future. For example, carefully planned two-age cuts with reserves of live trees could retain patches of live and dead trees with roost potential and also create desirable edge foraging habitat. Clearcutting “overmature” timber takes out trees that, with old age, would otherwise have the potential to become suitable roosts for Indiana bats. On the other hand, there may be a positive effect on the future availability of Indiana bat roosts if silvicultural practices like single-tree selection are effective in “releasing” residual trees so those trees are able to realize their full size potential and, ultimately, die.
In addition to carefully planned harvests, other activities that may produce suitable habitat for Indiana bats include prescribed burning, installation of ponds and ephemeral wetlands, and maintenance of narrow trails or roads through forests. Snag creation may occur via prescribed fires or as a result of damage during harvests, or through the more active method of girdling or injecting live trees with herbicides. Indiana bat populations are likely to benefit from snag creation only when snags are not common in mature forests. Male Indiana bats in Missouri use snags created by girdling, but we need more information to determine if such manipulative strategies are effective and when they are warranted. Buffers of mature trees left in riparian areas should provide suitable habitat for bats and reduce soil erosion, thereby improving local water quality and the quality of the available prey base. Connectivity among habitats is also important; for example, it is important to connect suitable roost habitat with quality foraging areas and to maintain travel corridors among potential roosting and foraging areas.
When suitable habitat remains, Indiana bats will be faithful to the same sites year after year. Thus, for all types of timber management, it is important to maintain a component of mature forest during any transitional stages. Evidence from across the species’ range suggests that Indiana bats are adaptable in terms of the species of trees and the types of woods they will use, but they must have an ongoing supply of large snags for roosts and surrounding forested habitat within which to forage. Without these, it is likely that a colony will abandon an area or forgo reproduction upon finding no suitable habitat in spring.
Forests are important to Indiana bats and, in Indiana, private woodland owners have an important role to play in maintaining suitable forested habitat for this species. With some creativity, a balanced approach, and long-term planning, management for timber can be compatible with management for viable Indiana bat populations in the Midwest and across the species’ range.
Joy O’Keefe is an assistant professor in the Department of Biology at Indiana State University. Her research investigates the roosting and foraging ecology of forest bats, with a focus on the effects of active forest management on habitat selection.