Stanley Gehrt

Wildlife Biologist

Ohio State University

During a three-summer study from 1997 to 1999, when Stanley Gehrt was a research biologist at the Max McGraw Wildlife Foundation in suburban Dundee, he and his colleagues used an ultrasonic detector to monitor bats at more than a dozen sites in Cook and McHenry counties. They found them surprisingly populous. Turns out that bats like the city.

Harold Henderson (HH): In your article in the March issue of Chicago Wilderness Journal you called bats “perhaps the most efficient predatory mammals.” What does that mean? How many fewer bugs do we have because bats ate them?

Stanley Gehrt (SG): A single little brown bat can catch 1,200 insects an hour. I don’t know of other mammals, especially terrestrial mammals, that can regularly catch as many prey per attempt as bats.

HH: How can I tell that there are bats in the city at all?

SG: Most bat activity occurs in the first hours after sunset. Right after the sun goes down is a good time to look, while the sky is still lit and birds have mostly settled down for the night.

HH: Don’t bats fly differently from birds anyhow?

SG: Yes, birds are stronger and more linear flyers. Bats rarely fly in a straight line, they tend to flutter and change direction constantly, and they have very quick wing beats.

HH: Since they’re wild animals preying on insects you’d think there would be more of them in the country than in the city.

SG: Not when you have rural areas that look like Illinois’ or those of other parts of the midwest. What little work has been done on bats’ urban ecology was done on the east coast or in Canada, where the rural areas are forested (most bats roost in trees). But in rural Illinois you have a landscape severely altered by human activity. It’s not ideal habitat. Many trees have been cut and the fields have been tiled and drained. It’s been totally altered into a monoculture of corn and soybeans. Compared to that you can see that an urban landscape might hold some promise for bats.

HH: Even small patches?

SG: Yes. The smallest patch we looked at was slightly less than 100 acres [roughly the size of the former Meigs Field], and it had tremendous usage.

HH: Does it matter what’s in the patch?

SG: We thought that if the bats were using the edge between woodland and open areas there’d be a big difference between mowed grass and restored native vegetation. We found no difference. If anything there was a trend toward the mowed areas. There was a tremendous drop if the woods were next to cropland. That was statistically significant.

HH: So given a choice, bats prefer woodlands next to mowed or restored parklands over woodlands next to farm fields.

SG: Yes. When we first found that pattern, I thought we had made a mistake–something must be wrong with our samples! We added monitoring sites in rural western McHenry County, yet the pattern was even stronger. Eventually we had to swallow our results.

HH: Given that bats are hard to see and impossible to hear, what kind of equipment did you use?

SG: The bat detector is a rectangular box, about nine inches tall, six inches wide, and only two or three inches deep, with a little microphonelike receiver on top. It detects the ultrasonic frequencies emitted by bats echolocating [as they fly] and any other ultrasonics. It was fun to point the bat detectors at televisions, computers, air conditioners and see what you could hear. Air conditioners are one of the worst. Put it right next to a car–no ultrasonics. This reminded us that there’s a different world out there that we aren’t aware of but bats are. They have a different hearing spectrum, so we don’t always know what they’re responding to.

HH: Are bats like frogs, where you can learn to recognize a species by its distinctive call?

SG: The bat detector divides the bats’ frequency a certain amount so that we can hear it. It matches their pitch variations, which are what we use to identify different species. The really deep-voiced bats like the hoary bats stand out, and the Myotis are really high-pitched. But there’s a whole bunch of species in the middle. For them you have to use the computer. The software we use enables us to play it back, and you can see the call on the screen. Unlike birds and amphibians, bats call at a constant rate. (When there’s a lot of activity in the area, it sounds like popcorn popping.) And their differences in frequency aren’t that great. On the computer screen you can make them look greater.

HH: So good wildlife habitat isn’t always what we expect?

SG: The take-home message for me is that the relation between urban areas and wildlife is more complicated than we think. Believe me, in terms of wildlife research, urban research is the least done.

HH: That’s not why people go into wildlife biology!

SG: As a wildlife biologist in Chicago studying mammals, I felt the list of potential questions and species to work with would be pretty short. This study shows how complicated it is and how much more work we have to do. Certainly some species are detrimentally affected by urbanization in every case. Some are always benefited. With bats, there are many variables at play. We expected bat numbers to drop off as we moved to the core of the city–not at all. We collected red bats from Michigan Avenue where there were no woodlands at all. Big browns roost in parking garages. No one ever looks there; they’ve just assumed they had no value.

Art accompanying story in printed newspaper (not available in this archive): photo/Lloyd DeGrane.