The die-off is affecting little brown bats and other species.Around 90 percent of hibernating bats in caves and mines in New York have died since last winter, in one of the worst die-offs to hit bat populations in the United States.

According to the New York Times, wildlife biologists are seeing the phenomenon in 15 caves and mines in New York, as well as at sites in Vermont and Massachusetts. Scientists say they are not sure what is killing the bats, which are found unusually thin and, in many cases, dotted with a white fungus.

The Department of Environmental Conservation and the U.S. Fish and Wildlife Service are working to determine if the bats are being killed by a virus, bacteria, environmental hazards or other causes.