Three summers ago I added a new wildlife-related structure to my ever-growing collection of bluebird, house wren, wood duck and kestrel houses and other nesting structures. I installed a bat house on the utility pole next to my house.
I erected the two-foot-tall, black-painted bat house about 12 feet high and hoped that it would soon become occupied by a few bats. Knowing full well that the box's occupancy might take a while, I was nonetheless disappointed when no bats roosted in it during the first summer, and doubly disappointed when no bats roosted inside the humble abode last summer.
Then came summer 2011. Just as I did the previous two summers, I occasionally monitored the bat house to see if any bats were inside. I did so by examining the ground underneath for bat droppings--a sure sign that bats were using the box. In addition, I'd gaze at the compartmentalized, underside of the structure, too; if I observed cobwebs spanning the compartments, then the likelihood of bats being inside was remote.
However, one day in late July, I again searched the ground below the bat box for droppings, or guano, as bat feces are also called. And this time, bingo! Bat poop! A few moments later I was partway up a ladder and peering inside the bat house with the aid of a flashlight and . . . there they were, hanging upside down, slumbering the day away -- one, two, three, four . . . eight total of two species; one big brown bat and seven little brown bats.
I was thrilled!
Bats are the only mammals in the world that can actually fly and are represented by many different species that occupy a variety of habitats around the world. Here in Minnesota, seven species of bats fly about on any given night throughout the state -- as already mentioned, the big brown and little brown bats, plus the Keen's myotis, eastern pipistrelle, hoary bat, silver-haired bat, and eastern red bat. More than 900 species occur worldwide.
All of Minnesota's bats are insectivorous, that is, their entire diet consists exclusively of insects. And though mosquitoes are indeed consumed, they are not the only insects sought by bats. Bats seem to prefer mostly soft-bodies insects, especially moths, as well as mayflies, midges, caddisflies, lacewings, leafhoppers, and of course, mosquitoes. Other invertebrate delicacies include a host of beetle and bug species and even spiders.
Watching bats capture insects is a joy. It has been estimated that a bat can capture and consume as many as 1,000 insects in just one hour, or anywhere from 6,000 to 8,000 insects, which include mosquitoes, each night. Perhaps these figures sound excessive, but anyone who has ever watched bats flying, darting, diving and abruptly turning, twisting and maneuvering on the wing as they hunt for prey knows that this is indeed a possibility. Bats are very proficient at filling their bellies.
Some people may have you believe that bats cannot see at night. The oft-used expression, "Blind as a bat" is a curious axiom given the fact that bats can fly and capture insects in total darkness without any difficulty whatsoever. And while bats can and do see with their eyes, prey is located primarily by echolocation.
Simply put, bats emit high-frequency sounds that travel through the air. When the sound waves encounter, say, a moth, the waves bounce back to the bat's ears. In this way a bat can home in on a flying insect meal by listening, basically, instead of seeing. These high-frequency pulses produced by hunting and flying bats are inaudible to our ears and can occur at as high a rate as 200 pulses per second. It's possible, I suppose, that the bat I caught on a memorable cast had confused my flying monofilament fishing line with that of a flying insect.
When a bat does zero in on a moth or other flying insect, it doesn't always capture the meal in its open mouth like some insectivorous birds do. More frequently, a bat scoops the insect into its tail or wing membranes like a baseball player fielding a grounder. Without missing a beat, the bat reaches down and picks up the insect in its mouth and feeds in midair. That zigzag, up-and-down flight pattern so typical of bats in the air shows bats searching, locating, chasing, zeroing in, capturing and feeding on insects. What's more, bats accomplish these incredible feats by producing sounds, at night and in the air.
The two species of bats currently occupying my bat house (little brown and big brown), are the two most common species in Minnesota. Little browns average about 3.5 inches long, whereas big browns average about an inch longer. While both species have similar diets, it's believed that little browns prefer moths and big browns prefer beetles.
And though bats are sometimes vectors of rabies, we have little to fear from bats. These insect-eating mammals are in fact very beneficial to humankind. For instance, a research biologist estimated that a colony of 150 big brown bats (each member can consume its own body weight in insects in one night!) can eat enough cucumber beetles in a summer to thwart egg-laying that could potentially equate to 33 million of their corn-damaging, root-worm larvae.
As such, before autumn arrives and frost begins to cover the ground white, take a moment and go outside at dawn and dusk to search the sky for friendly-flying-furry bats as you get out and enjoy the great outdoors.
Blane Klemek is the Bemidji area assistant wildlife manager, DNR Division of Fish & Wildlife. He can be reached at firstname.lastname@example.org.