Lead is a neurotoxin that has thwarted efforts to restore endangered California condors to their historic habitat. It's the major cause of death for condors and affects other scavengers such as bald eagles, golden eagles and turkey vultures.
IIRC, lead shotgun loads have been illegal out there for years. Never made sense to me. The amount of lead leached into the ground by stray/unrecovered bullets is negligible compared to pretty much any other source of lead. And that's ignoring the fact that harder shot like steel is much more likely to over-penetrate and/or not cause lethal wounds - which kinda defeats the whole "its to protect the animals" side of things.
And let's not forget that any competent hunter will try to remove the slug from the animal, and that lead poisoning from a left-behind slug takes years to kill (and requires a still-living body, obviously), and that's if there's even enough lead in the bullet itself to reach lethal toxicity levels. That alone means the environmentalist push is complete bullshit.