Tom Kunz has been studying bats throughout New England for more than four decades. In annual treks to remote caves, he and other researchers from Boston University capture bats, banding them to track their migration patterns and movement over the years. During trips to bat hibernacula—the bats' winter hideouts—he grew accustomed to cave walls covered in huddled masses of bats, tens of thousands strong. Aeolus Cave in Vermont, the largest bat hibernaculum in the northeast, has long been a winter home for more than 100,000 bats. Kunz regards them fondly, like old friends. What he sees today brings him to tears.