A couple of days after Christmas, Carl Haynie and his wife, Terry Risdon, made the three-hour drive from their home in Sammamish, Washington, to the same remote beach they’ve visited every month for five years. They passed through the Seattle suburbs, hugging the rugged coast to reach their isolated, one-kilometer stretch on the Olympic Peninsula. Haynie and Risdon are beach bird monitors, two of the more than 1,200 volunteers who patrol shores from California to Alaska, recording any dead birds that wash up in their survey zone. Haynie, a software developer and longtime birder, likens the outings to treasure hunts. “You never know what you’re going to find.” He’s spotted carcasses of such rare birds as Pink-footed Shearwaters and Black-footed Albatrosses, and deduced the identities of 40-plus species by a wing, head, or foot alone—sometimes all that’s left. But the pair had never seen anything like what awaited them that December day. Dark bumps peppered the...