Indiana Bat - Cover Requirements

Cover Requirements

Primary roosts used by Indiana bats are typically snags in canopy gaps and forest edges that receive direct sunlight throughout the day. Alternate roosts live or dead trees, generally located in the forest interior, that usually receive little or no direct sunlight. Weather, such as very warm temperatures and precipitation, appears to influence the use of interior alternate roost trees over primary roosts, as alternate roosts generally offer more shade and protection during inclement weather and extreme heat. However, this preference may fluctuate from season to season. Indiana bats moved to the alternate roost during periods of heavy rain and colder ambient temperatures during fall in Missouri, but chose to roost in the primary snag during inclement weather in the spring. These differences may be attributed to variation in the heat retention capabilities of the trees at different times of the year. Gumbert and others discovered bats from a maternity colony switched roosts more frequently in summer and autumn than they did in spring in an oak-pine forest in Kentucky. Indiana bats exhibit strong fidelity to individual roost trees from year to year if they are still suitable roost sites. Many trees are no longer usable after just a few years, while others may last as long as 20 years.

Another important factor relating to roost suitability is tree condition. Indiana bats prefer dead or dying trees with exfoliating bark. The amount of exfoliating bark present on a tree seems to be insignificant. Indiana bats show an affinity for very large trees that receive lots of sunlight. Typically, Indiana bats roost in snags, but a few species of live trees are also utilized. Live roost trees are usually shagbark hickory, silver maple, and white oak. Shagbark hickories make excellent alternate roosts throughout the Indiana bats's range due to their naturally exfoliating bark. Although Indiana bats primarily roost under loose bark, a small fraction roost in tree cavities.

Primary roosts are generally larger than alternate roosts, but both show a lot of variability. Females typically use large roost trees averaging 10.8 inches (27.4 cm) to 25.7 inches (65.3 cm) as maternity roosts. Males are more flexible, roosting in trees as small as 3 inches (8 cm) dbh. In a review, Romme and others determined that Indiana bats required tree roosts greater than 8 inches (22 cm) dbh, while Clawson suggested that roosts of 12 inches (30 cm) dbh or larger were preferred. The heights of roost trees vary, but they tend to be tall, with average heights ranging from 62.7 feet (19.1 m) to 100 feet (30 m). The heights of the actual roosting sites are variable as well, ranging from 4.6 feet (1.4 m) to 59 feet (18 m).

In addition to day roosts, Indiana bats use temporary roosts throughout the night to rest between foraging bouts. Limited research has examined the use of night roosts by Indiana bats, and thus their use and importance are poorly understood. Males, lactating and postlactating females, and juveniles have been found roosting under bridges at night. Some Indiana bats were tracked to 3 different night roosts within the same night. Night roosts are often found within the bats's foraging area. Indiana bats using night roosts are thought to roost alone and only and for short periods, typically 10 minutes or less. Lactating bats may return to the day roost several times each night, presumably to nurse their young. Pregnant bats have not been tracked back to the day roost during the night except during heavy rain. Because Indiana bats are difficult to track during their nightly movements and usually rest for such short periods of time, the specific requirements that Indiana bats need in a night roost, and reasons why night roosts are needed, are still unknown.

During spring and fall, Indiana bats migrate between hibernacula and summer roosting sites. In New York and Vermont, bats traveled up to 25 miles (40 km) between hibernacula and summer roosting sites in spring. This is a considerably shorter distance than what is seen in the Midwest, where bats may travel up to 300 miles (500 km). Many males remain close to hibernacula during the spring and summer rather than migrating long distances like females. Occasionally, they even roost within hibernacula during the summer. Males also roost in caves and trees during fall swarming. Few data exist for the roosting requirements of Indiana bats during spring and fall migrations; data indicate that requirements during these times are similar to summer needs in that the bats chose large trees with direct sunlight and exfoliating bark.

The ability for Indiana bats to find suitable hibernating conditions is critical for their survival. A hibernaculum that remained too warm during one winter caused a 45% mortality rate in hibernating Indiana bats. Bats generally hibernate in warmer portions of the hibernacula in fall, then move to cooler areas as winter progresses. During October and November, temperatures at roosting sites within major hibernacula in 6 states averaged 43.5 °F to 53.2 °F (6.4–11.8 °C). Roost temperatures at the same hibernacula ranged from 34.5 °F to 48.6 °F (1.4–9.2 °C) from December to February. Temperatures in March and April were slightly lower than in autumn at 39.6 °F to 51.3 °F (4.2–10.7 °C). The Indiana Bat Recovery Team discovered that Indiana bat populations increased over time in hibernacula that had stable mid-winter temperatures averaging 37.4 °F to 45.0 °F (3.0–7.2 °C), and declined in hibernacula with temperatures outside this range. Temperatures slightly above freezing during hibernation allow Indiana bats to slow their metabolic rates as much as possible without the risk of freezing to death or using up fat too quickly. Hibernating Indiana bats may also survive low temperatures by sharing body heat within the tight clusters they typically form. Bats awaken periodically throughout the hibernation period, presumably to eliminate waste or to move to more appropriate microclimates. This periodic waking does not seem to affect the survival of Indiana bats, but waking caused by disturbance can cause Indiana bats to use up large amounts of energy, which can cause them to run out of fat reserves before the end of winter, possibly leading to death.

One way in which caves retain low temperatures is through a constant input of cold air from outside the cave circulating in. Typically, the caves supporting the largest Indiana bat populations have multiple entrances that allow cool air from outside the cave to come in, creating a circulation of fresh cooled air. Gates that are meant to keep vandals out of caves have altered the temperature and airflow of hibernacula, resulting in population declines of Indiana bats at many major hibernacula throughout their range. Removing or modifying gates at some of these have given these populations a chance to rebound. Also, the bats seem to prefer a relative humidity of 74% to 100%, although it is uncommon for the air to be saturated. Relative humidities of only 50.4% have also been recorded.