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Beneficial Bats Are Nature's Best Bug-Eaters

Bat Gate at Millie Mine

August 17, 2006

The tiny bat is one of nature's most fascinating and misunderstood creatures. Measuring 3 to 5-1/2 inches in length from head to tail and weighing only 0.1-1.2 ounces, bats are nocturnal in habits and are most active at dusk and early in the morning, just before dawn.

They are the only mammals capable of sustained flight. The wing of a bat, which is totally different in structure from that of birds or insects, is formed from skin stretched over long, thin fingers that encompass the hind legs and often the tail.

Bats use echolocation to find and capture prey. They emit pulses of high-frequency sound (20-130 kHz) that bounce off nearby objects. The bats then use the echoes to determine the object's distance, size and shape. Superbly adapted to flying at night, bats can navigate in total darkness, avoiding the thinnest of wire obstacles in their path with ease.

Michigan bats feed on a variety of moths, flies, beetles and other insects. When feeding under normal conditions they can capture 600 to 1,000 mosquito-sized insects per hour.

Bat NettingThere are in Michigan nine species of bats, and much of what is known about them comes from studies of the two gregarious (that roost in large groups) species most commonly found here -- the big brown bat and little brown bat. Keen's bat is another gregarious species widely distributed in Michigan, and the tiny eastern pipistrelle is an uncommon year-round resident of the western Upper Peninsula.

The Indiana bat, a summer resident of the southern Lower Peninsula, is of special interest because of its endangered status.

More inclined to keep to themselves and consequently more difficult to study are the silver-haired bat and red bat, and the evening bat, which is strictly a summer visitor in Michigan. The hoary bat, the largest species, is considered rare but can be found throughout the state in summer.

For most species, males and females typically spend the summer apart from one another. They come together again in early fall when courtship and mating are initiated. After mating, they'll over-winter in moderately sheltered hibernacula, including caves, mine tunnels, and occasionally in hollow trees.

Hibernation is an adaptation for survival during the winter months, where there are no insects available for bats to eat. Though many Michigan bats will migrate to areas where it is warmer, it is not so much to escape the cold as to find a suitable place to hibernate.

Upon leaving their hibernacula in spring, females will form small groups and move to summer roosts where they bear and nurse their young. These nursing sites generally are used year after year. Most young are born from late May through early July, and are usually capable of flight within 21 days. Young bats typically reach adult size by late summer.

Bat tourEarly metallic mineral exploration in Michigan's Upper Peninsula resulted in the creation of thousands of vertical and horizontal mines and pits. These sites, after mining operations ceased, became trash dumps and hazardous places for curious explorers.

But these mines also offered optimum bat winter roosting sites because of their constant temperatures, around 45 degrees, and high relative humidity.

One prime example is the Millie Hill site in Iron Mountain. A special gate installed on the opening to the mine prevents people from falling into its vertical shaft, yet allows bats to come and go as they please.

"The Millie Mine is one of about 20 sites that have been protected with similar bat entrance grates in the Upper Peninsula," said Bill Scullon, wildlife biologist at Escanaba. "It is the first one to be developed as a bat interpretive site."

Scullon said the Millie Mine is a critical hibernating and breeding location for up to 50,000 bats -- one of the largest known concentrations of bats in the Midwest.

"Big brown and little brown bats from all over the region come here to hibernate during the winter months," Scullon said. "They migrate in from Wisconsin, Minnesota and Ontario, and perhaps even from the Lower Peninsula."

The bats start arriving at the mine in late August and early September. They remain in the mine shaft throughout the winter and begin emerging in late April and May.

"The best time to view bats is in September and early October, right at dusk, when the bats begin to emerge from the mine to feed during the night," Scullon said.

bat festiva;The mine has become an area attraction and for several years was the highlight of the annual Great Lakes Bat Festival held each summer at Iron Mountain. The popular event was held in cooperation with the Organization for Bat Conservation, which has hosted a similar annual event at the Cranbrook Institute of Science in Bloomfield Hills. The 2006 Great Lakes Festival was held earlier this month at Cranbrook.

"This year, it was decided to combine the two events into one larger event that could be held at a single location in the Great Lakes region," Scullon said. "Next year's festival is planned for the University of Indiana in Bloomington and, in 2008, we'll be back in the U.P. at either Houghton or Iron Mountain."

The Organization for Bat Conservation is dedicated to teaching people about bats and conserving bat populations and habitat. To learn more about these fascinating and beneficial members of Michigan's wildlife community, visit their Web site at

"The next time you look toward the sky at dusk, keep your eyes open," Scullon said. "You may just catch a glimpse of these tiny shapes as they wheel and dart in silent silhouette before the last sliver of light fades into night."

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