Missouri's Winter Bats

By Jeremy Kolaks | December 2, 2007
From Missouri Conservationist: Dec 2007

Missouri is known for its thousands of natural caves. Many of these caves are famous for their vast numbers of hibernating bats, including the endangered Indiana and gray bats.

Until recently, we believed that three species of bats usually not associated with caves spend their summers in Missouri but travel south to more favorable climates when the days get short and the nights cold.

Research, however, has shown that Eastern red bats (Lasiurus borealis), evening bats (Nycticeius humeralis) and silver-haired bats (Lasionycteris noctivagans) remain throughout the winter, having adapted to withstand the rigors of snow, ice and occasional prescribed fires or wildfires.

Red bats are by far the most common of the three species. They can be seen flying on warm winter evenings and nights as they forage over forest openings and urban parks, often taking advantage of insects attracted to lights when the temperature gets above 50 degrees.

Red bats are common throughout North America. They range from the Rocky Mountains eastward and from Canada to Mexico during summer months. During those months they roost in the foliage at the tops of deciduous trees, seldom, if ever, roosting in tree cavities or caves.

Research conducted by the Missouri State University biology department at Taney County’s Drury/Mincy Conservation Area and Carter County’s Peck Ranch Conservation Area indicate that these bats exhibit unique behaviors to cope with Missouri’s winters.

In 2003, when research first began, a red bat was captured on a warm late-winter evening. We fitted the bat with a radio transmitter, but soon after release the temperature dropped to below freezing. The next day remained very cold, and when we tracked the red bat we found it deep in torpor (a light hibernation) on the ground, nestled under the leaf litter. Since that time more than 50 red bats have been tracked to leaf litter roosts on cold winter nights.

Evening bats, found in the eastern United States and northern Mexico, also forage for insects over forest openings. They are known to roost in tree cavities and occasionally in buildings, but only rarely in caves.

The silver-haired bat can be found in every state including Alaska. It has been found most commonly roosting on or under the bark of trees, but also in tree cavities, open sheds or outbuildings and the occasional rock crevice.

Evening and silver-haired bats can also be seen in the winter, though there are fewer of them, and they can be hard to distinguish from red bats while flying at twilight. Instead of roosting under the leaf litter when winter nights get cold, both species stay above the leaves.

Studies show that evening bats use more living trees in the winter than summer, and that the height of surrounding trees are lower. They have never been found using the same trees during both seasons. These bats will leave their tree roosts during warm winter evenings to drink; however, there was no indication they feed during these brief excursions.

During the warmer months of the year, silver-haired bats commonly roost on or under the bark of evergreen trees. In winter, however, they root only in large deciduous trees with obvious cavities or crevasses that provide them protection from precipitation and cold temperatures. Although they are not as active at low temperatures as red bats, they do feed during winter.

The differences between summer and winter roosts of red bats are dramatic. During summer, red bats are found almost exclusively in shady areas at the top of the deciduous forest canopy. After leaves fall, however, they are found roosting in dead oak leaves lower in the trees, choosing locations on southern-facing slopes with maximum exposure to the sun and minimum exposure to cold north winds.

As temperatures approach freezing, they leave their leafy roosts and fly to the ground where they crawl under the leaf litter. Once again, they choose southern-facing slopes with exposure to the sun.

Red bats might leave their tree or leaf litter roosts if the afternoon temperatures rise above 45 or 50 degrees, and they might continue foraging during the first hour of darkness, even if temperatures fall to 40 degrees. This species feeds regularly during this time but only rarely is observed to drink. If the temperature stays above freezing, these bats will spend the night hidden in hanging dead leaves, but if it gets below freezing, once again they will find a spot under a protective covering of leaves on the forest floor.

It is this strange but normally successful strategy to cope with falling temperatures that led us to consider the impacts of using prescribed late winter or early spring burns in forest management.

Forest and wildlife managers have noticed bats flying out from under the leaf litter in front of prescribed fires and wildfires for many years prior to this study. However, over the past two years firefighters and biologists alike have been paying special attention while conducting fire activities. Since that time, there have been multiple confirmed sightings of bats flying out of the leaf litter ahead of, and even behind, fires. Interestingly enough, red bats appear to prefer areas that are under prescribed fire management over areas that are not.

This is not to say the fire doesn’t have a negative impact on bat populations. Further research is needed to quantify the direct effects of fire. It is possible that short-term negative impacts could pay future dividends in terms of the amount and quality of habitat available to Missouri’s winter bat species.

If you see bats flying, hanging in dead leaves, or under or emerging from leaf litter from November to March, please go to biology.missouristate.edu and click on “Missouri Bats,” which contains links to a survey that will help us gather more information about our winter bats.

Bats on the Fly

Some bat studies can be performed in the laboratory, but most research has to be carried out in the field.

We capture bats in fine-mesh nylon nets placed over gravel service roads or ponds. Each bat captured is assessed, weighed, measured, outfitted with either a radio transmitter or numbered band, and then released. Recaptured banded bats help provide population estimates. Bats outfitted with transmitters help biologists study their habitat requirements.

We use telemetry to home in on bats equipped with radio transmitters. Considering that red bats, for example, might travel more than 10 miles in one night over various terrains and multiple properties to get from their roost to sites where they feed, you can imagine the difficulty we would have tracking bats on foot or with a vehicle.

Fortunately, the Conservation Department has aircraft available to aid us in our research. The use of aircraft significantly reduces the number of man-hours required to complete our work and helps ensure that information can be gathered in a timely manner.

The Department’s helicopter is outfitted with a radio receiver that can locate animals that travel too far to be tracked any other way. Currently, and in the past, Department aircraft have been used in studies on deer, turkey, ducks, doves, otters and even catfish. Without this resource we would not be able to manage Missouri’s fish, forest and wildlife as efficiently and effectively as we do.

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