I’ll make one prediction about this coming summer: the floor of the dry storage above our garage will become covered in bat guano. How do the culprits of this mess – likely little brown myotis bats – actually get into the storage area? And (I ask as we gear up in face masks for the annual cleanup) just what it would take to bat-proof the place? And if we succeed, where will our little friends roost?
They are our “friends” because of the job they dutifully perform throughout the growing season: pest control. Bats are amazing exterminators of everything from moths to beetles, flies, and my Enemy #1: mosquitoes. A single bat can eat a thousand mosquito-sized insects in a single hour. “They fill a nice niche of feeding at night, when the other insect eaters aren’t necessarily out,” says Scott Darling of the Vermont Department of Fish & Wildlife.
In Vermont and New Hampshire, we have nine different bat species. Six of them spend the entire year here: little brown myotis, big brown, northern long-eared myotis, eastern pipistrelle, the federally endangered Indiana, and Vermont- and New Hampshire-state-threatened eastern small-footed myotis. Three migrate south in the winter: the silver-haired, hoary, and eastern red bat. The ones who stay here hibernate in undisturbed caves as well as in mines, which the Connecticut River valley is particularly rich in, thanks to its extensive copper-extraction history.
Every spring, from early April to late May, bats emerge from their hibernacula and seek their summer habitats, which they often use year after year, says Darling. Females, who mated in the fall, when large numbers of both genders were gathered at cave entrances, begin to congregate in maternity colonies to begin gestation of their young. They wait to begin growing their young until spring, when nutritious forage is available to fuel this energy-intensive endeavor.
Females of all bat species seek shelters for their maternity colonies. Many choose man-made structures, says Stephen Trombulak, a biology professor at Middlebury College. They end up in barns, attics, or in my dry storage, squeezing through impossibly small openings. “One thought on why the little brown bat has done so well in the last 100 years is the availability of so many nice, human-made habitats,” says Trombulak, mentioning in particular the many abandoned barns dotted throughout our landscape. Others prefer natural cover, especially in bottomland woods, such as under the bark of large, dead or dying trees, or in live trees with naturally loose bark, such as shagbark hickory.
Increasingly, areas hosting these bat summer homes are being converted to bat-unfriendly habitats, such as parking lots or developments. Even if forest remains, it is often made inadequate for bats by the type of management performed in it. “In a forest setting,” says Darling, “you can enhance habitat by maintaining large, dead, and dying trees – the ones often removed in intensively managed woodlots.” He recommends a target of four or more of such trees per acre. In the White Mountain National Forest, timber management plans recommend the creation of small openings in the forest and the retention of older hardwood trees to encourage healthy bat populations.
Another threat to bats, Darling says, is the health and integrity of aquatic communities, which are a major source of bat food. In addition to maintaining dead trees as part of your forest management plan, Darling says, “it is also important to maintain riparian buffers along wetlands and streams for aquatic invertebrates.”
The exclusion of bats from human structures is an additional threat to bat populations. If you don’t have good forested habitat for bats, but you would also prefer to avoid the yearly biohazard-suited cleanup of your attic, don’t despair. Build a bat box, preferably the year before you attempt bat exclusion. Bat boxes (and larger bat sheds) are fairly easy to make, and plans and even finished houses are available from websites such as www.batconservation.org (along with placement instructions, which if followed will help ensure colonization of your bat house). Be sure to wait until an evening in late summer, when young bats are grown and out for the evening, to plug up entrances to areas you’d like to exclude bats from.
Considering the threats to bat populations – loss of habitat, destruction of food sources, and purposeful exclusion from roosting sites (not to mention pesticides and the loss of winter hibernation sites) – and evidence pointing to a general decline in populations of most bat species compared to historical levels, these insect warriors could use a helping hand. “Bats really are living on the edge,” says Darling. “It takes an awful lot of energy to fly; they, they spend six months in a cave, and they have to make a living and bear young very quickly.”