Rafinesque’s big-eared bats often roost in an assortment of human-made structures, such as bridges, dilapidated barns and abandoned houses, in the southeastern United States. But their natural roosts – large hollow trees – are fast disappearing from managed forests, forcing many of these rare bats into a variety of artificial alternatives. Now a team of Mississippi researchers has discovered a novel – and virtually free – way to help nature provide hollow-tree roosts.
Wildlife biologist David Richardson of the U.S. Fish and Wildlife Service, graduate student Candy Stevenson and Professor Jeanne Jones of Mississippi State University are surveyingbig-eared bat roosts in the bottomland hardwood forest at the Noxubee National Wildlife Refuge in east-central Mississippi. The work is supported by Bat Conservation International, the U.S. Fish and Wildlife Service, U.S. Geological Service and Mississippi State.
For several years, the researchers have been repeatedly inspecting a sample of about 650 randomly located hollow trees to determine whether, when and how Rafinesque’s big-eared bats use them. Each cavity has been inspected at least eight times over two years.
Only 80 of the trees have been observed with bats, mostly single individuals – presumably males, which tend to roost alone or in small groups. Very old, very large hollow trees are clearly the most important for these bats, but the team has documented big-eared bats using cavities in trees as small as 16 inches (40.6 centimeters) in diameter with hollows just seven feet (2.1 meters) high.
Maternity colonies, located by following radio-tagged female bats, are typically found in a few ancient cypress trees with cavities that measure 60 inches (1.5 meters) or more in diameter. Sometimes the radio signal clearly indicates the presence of a bat within a tree, but the only opening is 60 feet (18 meters) or more above the ground. The challenge then is to find a way to easily examine the cavity without having to climb the tree.
Their solution was to cut a small opening, typically about 12 by 18 inches (30 x 45 centimeters), into the base of the tree. They simply plunge the bar of a chainsaw through the tree trunk to create a “removable window,” which can be reinstalled to maintain the internal environment. These portals provide a unique view of maternity colonies and seem to have virtually no impact on the roosting bats.
When the researchers identified hollow trees suspected of having unseen upper entrances, they used chainsaws to cut viewing portals that let them examine the cavities, then simply replaced the cut-out chunk of wood to close the window.
But some of those trees had no opening at all to allow bats to reach what otherwise seemed an attractive roost. With those trees, the team simply left the portal open. Within weeks, we observed a colony of southeastern myotis in a cottonwood tree with the cutout, while Rafinesque’s big-eared bats settled into newly opened cypress and hickory hollows.
This concept needs further research, but initial observations are extremely promising. The biologists find the prospect that simply creating access to existing tree hollows can provide long-term day roosts and perhaps even maternity sites very exciting.
BCI members can read the whole story of the almost-natural hollow-tree roosts for rare bats in the Winter 2007 issue of BATS magazine.