Even an expert hiker can take a tumble. Mark Kelley is more seasoned than most, so on a recent mild, sunny day on a flat trail, he was quick to point out an icy patch. “Don’t fall,” he cautioned.

On a summer night just before sunset on the summit of Mount Mansfield, biologist Nathaniel Sharp and a crew of volunteers are getting ready to try to catch Bicknell’s thrush. “If you even give, like, ...