In the run-up to Halloween, the Thetford and West Fairlee Conservation Commissions presented a well-timed talk on bats – those night-time flyers whose iconic silhouettes grace many a Halloween decoration. The speaker was Jesse Mohr of Native Geographics LLC who shared his knowledge of Vermont bat species and their ecology.
Bats became associated with Halloween in medieval Christian Europe, where their night-flying habit and leathery wings became an allegory of the devil. A bat entering a house was thought to forewarn of a death. Other misconceptions include the idea that bats are flying mice that carry rabies. In fact, bats (classified as order Chiroptera) evolved some 50 million years ago in a completely separate lineage from mice (order Rodentia). Less than 1% of bats carry rabies, and infected bats are not overly aggressive. Bat bites are very rare; however, a grounded bat might be rabid and should not be handled without thick, protective gloves.
Even so, why care about bats?
By eating insects, bats save US agriculture billions of dollars each year in pest control. According to the US Geological Survey, the value of this service is estimated at over $3.7 billion and possibly as much as $53 billion per year. That does not include the volume of insects eaten by bats in forests that benefits the timber industry or the critical importance of bats as crop and plant pollinators.
Bats are the only mammal capable of true, powered flight. Others, like the flying squirrel, merely glide, which is controlled falling. While birds can fly faster than bats, the bat is the winner in terms of maneuverability. The contour of a bird’s wing is defined by the feathers, and the wings usually mirror each other in shape. A bat’s wings consist of skin stretched between long finger bones. Thus they can bend their fingers to crumple one wing while the other is outstretched. This allows acrobatic feats, such as hard turns and somersaults to perch upside down on a cave ceiling or enfolding a flying insect with one wing and sending it to the bat’s mouth while in flight.
Bats are not blind. However, they do not rely on sight to fly in the dark. Instead they use echolocation, sending out a very rapid series of high-pitched chirps and listening for the echoes bouncing off nearby objects. Their chirps can be as loud as 120 decibels, which is equivalent to a smoke alarm going off 10 cm from your ear. One of our local bats, the Little Brown Bat, can chirp at that intensity. We don’t hear it because it is ultrasonic, meaning outside the frequency range that human ears can detect.
Bats are not deafened by their own calls because they are able to disconnect the bones in their middle ear, reducing their hearing. They reconnect in time to listen for the echo of each chirp. As bats close in on their flying prey, the number of chirps increases to 160-190 per second. In fact, the muscles in their larynx can contract up to 200 times per second, which is 100 times faster than a typical muscle.
With all these marvelous adaptations to life on the wing, how is it that bats are in trouble? Jesse Mohr explained that one reason is the epidemic of White Nose Syndrome (WNS), a recently introduced fungal disease that kills hibernating bats – more than 6 million so far. In particular the Little Brown Bat, once numbering in the hundred of thousands in Vermont, is now practically extinct and on the Vermont Endangered Species list. In winter, bats hibernate together in deep caves or mines, where the temperature stays around 50 degrees. Because bats investigate more than one cave at the onset of winter, they carry the disease from one place to another. Human visits to caves may also contribute to the spread.
Wind turbines are another contributor to bat decline. Migrating bats (and birds) are struck by the turbine blades or die from lung explosion or implosion due to the huge air pressure changes encountered close to a turbine. Wind energy facilities affect more than 50% of North American bat species. The bats most at risk are the larger, tree-roosting species.
The leading cause of bat decline, however, is habitat loss. This includes the clearing of land, especially during June and July when bats are breeding. In the summer, bats frequently roost in trees, including under the peeling bark of dead or dying trees that landowners tend to eliminate. This is particularly true of the federally threatened, Vermont endangered Northern Long-eared Bat. Bats also roost in rock piles and can be displaced by quarrying operations. Urban and suburban sprawl, industrial development, and agriculture remove foraging areas, a result especially harmful to migrating bats that follow the same route year after year. Human intrusion into mines and caves during hibernation also takes a toll. Cats kill bats, too.
Bats are slow to breed, which is why they are not recovering from White Nose Syndrome. Mature females have one pup, or sometimes two, per year. The females cluster together in maternal roost colonies to warm the developing pups. They may use a hollow tree or a barn or attic. When these roosts are disturbed or destroyed, the bats will scatter into less suitable roosts and the pups may not survive. Breeding in future years is also jeopardized. The most important thing we can do to help bats is to protect maternal roosting colonies.
Jesse Mohr explained that we have nine species of bats in Vermont. Six species are cave bats that hibernate in relatively local caves and mines during the winter: Big brown bat, Little brown bat (state endangered), Indiana bat (federally and state endangered), Tri-colored bat (state endangered), Northern long-eared bat (federally threatened and state endangered), and Eastern small-footed bat (state threatened). Three species are migratory bats that fly south to warmer climates for the winter and roost in trees during the summer: Silver-haired bat, Hoary bat, Eastern red bat.
Jesse also gave some pointers on what to do if you think you have bats living in your house, including a comprehensive guide to Best Management Practices for excluding bats from homes provided by Vermont Fish and Wildlife.
In addition, VT Fish & Wildlife is documenting bat colonies and appreciates it when the public reports them.
Note that excluding bats is season-dependent. Blocking bats out between mid-May to mid-August will prevent mothers from returning to their pups, so the pups will starve.
Jesse Mohr was not enthusiastic about bat houses as alternative roosts. They are frequently designed by non-biologists and go unused. However, he did provide a resource for a biologist-approved bat house design.
Other ways to help bats include keeping cats indoors and planting native flowers, trees (especially oaks that support many species of moths), and shrubs that are host plants for native insects. Consider replacing part of the lawn with a wildflower meadow (similar to a pollinator planting).
At the landscape scale, bats forage in forest clearings and travel ways. Clearings with irregular edges offering varied heights of trees and shrubs are better than clearings with sharply defined edges. A large pond provides a place for bats to drink on the wing, as they cannot walk on land. Their legs are used only for hanging upside down.
Preserve that dead tree – bats roost in tree cavities and under peeling bark.
Don’t use pesticides. Insects, the sole food of bats, are declining precipitously under an onslaught of factors including pesticides, loss of habitat, night-time electric lights that disrupt breeding behavior, and climate change. Whatever harms insects harms bats and birds, as well.
With their various “superpowers” like hunting in the dark by echolocation and their aerobatic agility, bats are neither evil nor spooky. Rather they are exquisite examples of evolution. A single bat will eat 2000 insects per night. And the next time you enjoy a shot of tequila, thank the Mexican long-nosed bat that pollinates agave plants.