The magnificent California Condor has a nearly ten-foot wingspan—and in comparison, the number-one threat to the endangered raptor is practically microscopic. It lurks unseen in deer, elk, and other carcasses the birds scavenge. Ingesting less than a gram is enough to seriously sicken a condor; in the last 25 years, it has felled at least 76 of these giants. That threat is lead, largely derived from ammunition. Now, as a 2013 law goes into full effect in the bird’s namesake state, that menace is in retreat. As of July 1, it is illegal to use lead ammunition to hunt any game anywhere in California. Since the law was passed, the state has slowly phased out lead bullets, which rupture, often into hundreds of tiny pieces, when they hit a target. Then, when condors scavenge carcasses or guts left behind by hunters, they ingest these pieces, too; the toxic metal causes the birds to starve to death because they can't hold any food down. Even with condor-hunting banned...