Especially worrisome, adds Greg Johnson a biologist with Western Ecosystems Technology, is that no detailed studies have been conducted in Texas, Oklahoma, Arizona or New Mexico, prime development regions where significant numbers of bats occur.
Bat research became a priority after 475 bat carcasses were found in routine monitoring following the 2002 construction of the 44-turbine, 66 MW Mountaineer Wind Energy Center in West Virginia. In response, BWEC was formed by interested parties including the American Wind Energy Association (AWEA) and Bat Conservation International.
Last year, the co-operative conducted research at the Mountaineer project and at the 30 MW Meyersdale wind farm in Pennsylvania. Both projects use NEG Micon 1.5 MW turbines and are owned by FPL Energy. BWEC's studies suggest that an average of 38 bats per turbine were killed at the projects and that a majority were killed on nights when average wind speeds and power production were low.
AWEA's Tom Gray says the industry is concerned. "The wind industry is committed to, and has demonstrated, continual innovations leading to greater protection of the environment and wildlife," says Gray. "We'll continue to do that as we work to understand the bat issue and find solutions to the problem."
Arnett says the next step is to determine whether feathering rotor blades (reducing or stopping energy capture) can reduce bat kills, but that no wind plant owner is willing to allow that research. In lieu of feathering experiments, Arnett says, the group will study whether the number of casualties can be predicted by measuring the number of bats at a site before construction and carcasses after operation. Another area of study is whether bats can be prevented from approaching turbines, perhaps using acoustic deterrents.
Johnson, who reviewed 50 scientific papers and studies on bat and turbine interactions, says bat deaths mainly occur in the heavily forested eastern US, where deaths averaged 46 per turbine per year. In the Midwest and Northwest, annual bat deaths averaged less than two per turbine, possibly because turbines tend to be sited in open habitats.
Based on all available evidence, Johnson says, most fatalities seem to involve long-distance migratory tree bats during their fall migration. Ninety percent of fatalities took place during the fall migration, roughly mid-July through the end of September, and 83% of all fatalities involved migrating bats. That could pose problems for the endangered Indiana bat. Although no threatened species have been documented at wind farms, some conservationists worry that the Indiana bat's migratory pattern takes it through prime wind sites.
Scientists continue to ponder why bats, with their highly developed echolocation abilities, collide with turbines at all. In the laboratory, bats demonstrate the ability to navigate through cluttered obstacle courses and easily avoid moving objects. One theory is that a high-pitched whine from the turbines may interfere with the bats echolocation abilities or even attract the creatures. Other possibilities are that bats may be caught up in wind shear created by the blades, or that they may be attempting to roost on stationary blades and towers, behaviour one researcher observed.
To avoid or reduce bat mortality at future wind farms, Johnson says, it is imperative to determine bat migration corridors and to predict risk to bats prior to construction. Arnett concurs. "It's not a matter of if the fatalities continue at such high rates, will bats be able to recover," he says. "Bats simply cannot recover from these mortality rates." By the time people comprehend the magnitude of the problem, he adds, "it might be too late."