In 1987, wildlife veterinarian Mark Pokras was in his office at Tufts University in Massachusetts when a colleague from New Hampshire called. “I’ve got a dead loon here,” she said. “If I bring it down to you, can you tell me how it died?”

Understanding what’s killing wild animals is often the first step to saving them, and over the course of his career, Pokras has necropsied everything from whales to hummingbirds. Yet this was his first loon—in part because common loons (Gavia immer) had only recently begun repopulating New England after being nearly extirpated by 300 years of hunting, pollution, and habitat loss. European settlers so disliked these “ill-shap’d” birds that nature writer Henry David Thoreau described his neighbors on Walden Pond in Massachusetts shooting them just for fun. By the early 1900s, common loons—which winter on the coast and spend summers nesting along inland lakes—had disappeared from their breeding grounds in Massachusetts and much of New Hampshire, and been reduced to a fraction of their former abundance elsewhere in New England. Places with names like Loon Pond didn’t see nesting loons for more than a century.

Cutting into the bird, Pokras discovered that it had suffered from lead toxicosis, more commonly known as lead poisoning. Loons eat pebbles to help digest food in their gizzard, and this one may have mistaken a lead sinker left behind by a fisherman for a pebble, or perhaps eaten a fish with a lead sinker in its body. In Pokras’s X-ray, the sinker showed up as an unnaturally round ball amid a mess of partially digested fish and shellfish. After the bird ate it, the lead would have leached toxicants into the bloodstream, causing impaired vision, gastrointestinal distress, neurologic issues, and ultimately death.

In an era when many species are declining because of multipronged, seemingly intractable problems, the solution to protecting loons is relatively straightforward. Anglers simply need to swap their old lead jigs and sinkers for tackle made from tungsten, steel, tin, or bismuth. Given loons’ immense popularity, you might think that would be an easy sell. But although conservationists have tried educating the public for decades—and although Maine, New Hampshire, Vermont, and Massachusetts have laws regulating the use and sale of lead fishing tackle—lead is still responsible for around 25 to 30 percent of loon deaths in most states and provinces, and until recently, nearly 40 percent in New Hampshire. Why?

The answer, at least in the United States, is entangled with gun rights.