When thousands – or millions – of bats gather in a cave or mine, figuring out how many are in there can be a daunting challenge. Yet, accurate counts are critical in determining the status of bat populations, especially endangered species such as the Mexican long-nosed bat.
New technology may hold the soluton: Thermal imaging and computer visualization seem to produce more accurate counts with less disturbance of roosting bats.
Infrared thermal-imaging cameras were used to count Mexican long-nosed bats (Leptonycteris nivalis) emerging after sunset from Emory Cave in Big Bend National Park in West Texas. The scientific team included Loren Ammerman of Angelo State University in San Angelo, Texas, and Thomas H. Kunz (a member of BCI’s Scientific Advisory Board) and Nickolay Hristov of Boston University.
The project was funded in part by BCI’s North American Bat Conservation Fund and the National Science Foundation and undertaken with the cooperation of the national park and Resource Manager Raymond Skiles.
In the traditional bat-counting method, in use for decades, you enter the roost and count the bats in a manageable space (typically a square foot or a square meter). Then you measure or estimate the total area where bats are roosting. A rough total is obtained by multiplying the square feet of roosting space by the number of bats per square foot.
The accuracy of this method depends heavily on the experience of the counter and on a relatively discrete roosting area. Estimates sometimes vary widely.
Mexican long-nosed bats are listed as endangered in Mexico and also in their narrow range in the Southwestern United States. A colony of the nectar-eating bats, which migrates north from Mexico in the spring, spends summers at Emory Cave. These bats are critical pollinators of agaves and many cacti in arid regions, but they are reportedly in decline. Reliable population data throughout their range are not available, however.
The researchers turned to thermal-imaging video cameras, which are effective even at night since they detect heat from the bats’ bodies. Last summer, the team recorded the emergence of bats at Emory Cave on each of six nights. The recordings were played back on a computer and the bats were manually counted. Traditional, surface-area counts were also made on two mornings.
The scientists determined that, at their peak in July, there were at least 2,472 to 2,874 Mexican long-nosed bats in Emory Cave.
The team concluded that infrared thermal imaging is a more accurate and reliable means of estimating the size of this colony size than the traditional method, but it’s also much more expensive and time-consuming. This is a developing technology worth exploring.
BCI members can read the whole story on the use of new technology for counting bats in the Summer 2006 issue of BATS magazine.
You can support research projects like this one throughout North America by contributing to the North American Bat Conservation Fund. Please contact BCI’s Department of Development at email@example.com.