On a hot summer day, in July of last year, Inga Sidor found herself gazing down at a dead Common Loon that lay sprawled across her necropsy table in the New Hampshire Veterinary Diagnostic Laboratory at the University of New Hampshire. In her decades-long career as a veterinary pathologist, Sidor has performed necropsies on hundreds of loons. In fact, she first learned and practiced her trade on the large waterbirds, unlike most others in her field. Even so, this case struck her as unusual. The dead bird looked otherwise healthy: From the outside, it showed no signs of injury from boat strikes or gunshots, and an X-ray revealed no lead fishing weights in its gizzard. Right away, Sidor had ruled out the most common causes of loon deaths in New England. It was also rare to have such a fresh carcass on the table. Thanks to a vast network of researchers and volunteers, Sidor sees nearly every dead loon found in the state, but they typically arrive half-decomposed and...