Illinois’ bat species are ferocious predators of insects, including crop pests. Though tiny themselves, bats’ massive appetites for these undesirable insects help save farmers money on pest control measures. It is estimated that bats saves corn farmers about 1 billion U.S. dollars globally!
Yet their efforts are often overlooked and go underappreciated.
Bats are facing multiple threats in Illinois—habitat loss, poisoning by pesticides via the insects they eat, harassment by humans who misunderstand them, and death by wind turbines and White-Nose Syndrome.
With six of Illinois’ 13 bat species listed as threatened or endangered, it’s important that we try to minimize threats to bats.
To that end, I’ve been part of an Illinois Natural History Survey (INHS) research team that’s traveled the state for the last five years searching for signs of bats in, under, and around bridges and culverts—structures that bats often use as roosts.
Our surveys are done in association with the Illinois Department of Transportation (IDOT) to help IDOT staff avoid harming bat populations while repairing, cleaning, or demolishing bridges and culverts.
Bats and bridges
So, why do bats use bridges and culverts? These structures provide safe and reliable roosts and are abundant across the landscape.
Bridges and culverts also frequently cross bodies of water, such as streams and rivers. Bats use these water bodies as travel corridors and a water source for drinking on the fly. Additionally, there’s often ample food available, as many insects congregate near water.
The underside of a bridge offers an assortment of safe spots where bats can roost beyond the reach of predators.
Bats will roost in the open, hanging by their toes from rough concrete surfaces, the edges of metal I-beams, and other materials. They can also use spaces that were built into bridges, like expansion joints, or new areas that are created as bridge parts erode over time.
Searching for bats
INHS scientist Jocelyn Karsk and I are part of the Biological Survey and Assessment Program at INHS. Together with numerous field technicians, we have surveyed hundreds of bridges and culverts across Illinois looking for bats and/or signs of bats.
To do so, we examine the open areas under bridges to see if any bats are roosting there. Sometimes we have to use binoculars to get a better view of larger bridges, which can be over 20 feet above the ground.
We use high-powered headlamps or spotlights to illuminate and check cracks, crevices, gaps, holes, or anywhere a bat might hide.
We also search for signs of current and past bat occupancy on the bridge structure and on the ground under it, including staining caused by bat urine and/or guano (bat poop).
In 2017 and 2018—the most recent years we have tabulated—our crew assessed 200 bridges and 57 culverts. We observed more than 447 bats in 23 structures (20 bridges, 3 culverts; 8.9%) and found evidence of bats roosting (guano or staining) in 39 of the structures (36 bridges, 3 culverts; 15.2%). Not all structures that had bats had guano or staining, especially if just a single bat was found.
A number of Illinois bat species are known to use bridges and culverts as roosts, but we have only observed two species to date during our surveys.
The vast majority of the bats we found were big brown bats* (Eptesicus fuscus), which often roost in manmade structures like attics, barns, abandoned buildings, and bridges.
The big brown bat is found throughout Illinois and is one of the state’s most common species. It is the second largest bat in Illinois, weighing up to a whopping 0.9 ounces (equivalent to about 4½ U.S. quarters). Despite its small size, 25 big brown bats can consume about a pound of insects in a single night!
We have also spotted a few tri-colored bats (Perimyotis subflavus; formerly called the eastern pipistrelle).
The tri-colored bat is one of Illinois’ smallest bat species, weighing only 0.1 to 0.3 ounces (between a penny and 1½ U.S. quarters). Though found throughout most of the state, the tri-colored bat is not observed as frequently as the big brown bat. It also roosts in manmade structures. When roosting on trees, it will hang with a group of dead leaves at the end of a branch.
And the surveys continue
Our research team will continue assessing IDOT bridges and culverts for bats to help ensure these tiny mammals (and their big, ecologically and economically important appetites!) remain safe in the years to come.
* Most of the bats were identified by photos. Some of the bats may have been evening bats (Nycticeius humeralis), which look like smaller versions of big brown bats; however it is likely that most, if not all, were big brown bats as they are more common throughout Illinois than evening bats.