Forests for the Bats Part II: Managing Roosts and Hibernacula
Bats of the eastern US are in trouble. Millions have succumbed to White-nose Syndrome in the past decade, which can kill 90-100 percent of bats that hibernate together in caves over winter. Our bats have been declining for decades before White-nose Syndrome began spreading throughout the northeast, however. Their reliance on forests, outlined in Forests for the Bats Part I, means that they are just as susceptible to land use changes as the myriad other species of the mid-Atlantic which are challenged by habitat losses. Bats are especially sensitive to deforestation and forest mismanagement, but with a little consideration most landowners can do their part to support local populations which are in critical need of help. Protecting roost sites and overwintering sites, or “hibernacula”, is a great place to start.
The best thing the average landowner can do for bats is provide a sustained supply of roost trees. Most bats of the mid-Atlantic require roost trees in the summer where they can rest, take shelter, and raise young. The best roosts are tall, large-diameter trees that receive ample sunlight and have space beneath bark for bats to squeeze under. Recently dead trees, or “snags”, which have bark that is somewhat loose from the tree but yet not falling off, are primary roost trees. Some species, such as shagbark hickory (Carya ovata) and white oak (Quercus alba) provide space beneath bark while still alive. Snags will only accommodate roosting bats for a few years, so large shagbark hickories and white oaks that will stand for decades are very valuable resources. The bigger the better, but even small trees can be valuable; bats have been observed roosting in trees as small as three inches in diameter.
Inventory possible roost trees on your property and retain them when planning any cutting of trees. Roost trees on forest edges and ridgetops receive more sunlight and are even more valuable, and should be retained if at all possible. Creating snags is also an option if no suitable roost trees currently exist; a landowner can girdle trees that receive ample sunlight. Be sure to control invasive species around the site beforehand; killing the tree will also increase light to the forest floor, which is a risky invitation to uncontrolled invasive plants. Artificial roosts can be very helpful if there are no natural roosts nearby. The National Wildlife Federation has an excellent guide to creating and placing artificial roosts. Most bats require multiple roosts in an area, so the more roosting options they have, the more useful your property will be for them. Bats will often return to the same set of roost trees each year, so if you’ve got good roosts expect to have them back.
Eastern forest bats hibernate through the winter to survive the lack of insects available to eat. Caves are primary hibernacula because they maintain a regulated temperature and are free of perturbations that would rouse bats from hibernation. Waking mid-winter is dangerous because it causes a huge spike in energy that’s expended. There’s no extra food around, so a bat can’t make up for the reserves that were lost by bringing its metabolism and body temperature back to normal. If they don’t have enough energy reserves left to make it through the winter, bats will starve to death. Bats wake up on their own every once in a while throughout the winter, but excessive perturbation, including irritation from White-nose Syndrome, can result in depleted fat reserves and eventually mortality.
Protecting hibernacula is essential to supporting bat populations. Caves or mine entrances that are publicly accessible should be fitted with grates during the summer to exclude humans but allow easy bat use. While cutting trees can improve breeding and foraging habitat, maintaining a heavily forested, undisturbed buffer around hibernacula is critical. Reducing tree cover too close to a hibernation site can change its microclimate or hydrology, possibly making it unsuitable. The federal standard is no cutting of trees within ¼ mile (roughly 4.5 football fields) of a known hibernacula. If there are caves, cliffs, or large rock formations within heavy forest cover on your property, you may hold the rare resource of a bat hibernacula and have the opportunity to do an immense service to bat populations within hundreds of miles.
Protecting and managing roost trees and hibernacula are critical to support bat population growth and winter survival. But bats still have to eat! Learn about how to improve foraging habitat in Forests for the Bats III.