More than 600,000 bats were killed by wind turbines across the United States last year, with the highest concentration of kills in the Appalachian Mountains, according to new research. In a paper published in the journal BioScience , University of Colorado biologist Mark Hayes used records of dead bats found beneath wind generators and statistical analysis to estimate how many bats were struck and killed by generator propellers each year. "Dead bats are being found underneath wind turbines across North America," Hayes wrote. "This estimate of bat fatalities is probably conservative." The new estimate is among the highest yet for generator-related bat deaths. Previous studies have calculated mortality rates of between 33,000 and 888,000 a year. The bat deaths were calculated on a per megawatt basis, and the highest rates were associated with generators in the Appalachian Mountains-Buffalo Mountain, in the state of Tennessee, and Mountaineer, West Virginia. Hayes said his estimates were conservative for several reasons. Little information on bat mortality was available for wind generators along the Sierra Nevada ranges and Rocky Mountains, he wrote, and scavenging animals likely carried away a percentage of dead bats before they could be counted. Hayes also said that if a range of bat deaths were listed by a facility, he used the lowest one for his calculations. There are 45 known bat species in the continental United States, but biologists do not have a firm handle on their total population. Experts say the animals' small size and nocturnal habits make them difficult to survey. Nonetheless, biologists suspect their numbers are decreasing because of changing climate and diseases such as white-nose syndrome. Even under the best circumstances, bat populations grew slowly, as they give birth to one pup per year, and the mortality rate for young bats was high, Hayes said. While they are not generally beloved by the American public, bats perform two highly valuable services: they eat enormous amounts of flying insects, and they help pollinate crops like peaches and avocados.