The bald eagle that came into wildlife rehabilitator Belinda Burwell's care last month, just as the hunting season was coming to a close in North America, was a shadow of its former self.

The stiff and wobbly bird clung to life but showed distinct signs of lead poisoning, likely from scavenging the remains of big game left by hunters who killed their prey with lead bullets.

"She couldn't walk, couldn't fly," said Burwell. "If she tried to move, she would fall over, she would stumble."

Environmental groups say 20 million birds die worldwide each year from eating bits of lead in animal carcasses, because many US hunters use lead ammunition which leaves 3,000 tons of toxic fragments in gut piles and unclaimed kills.

The dangers of lead have been well known for decades, and steps have been taken to prevent human consumption by removing it from paint, gasoline, pipes, children's jewelry and more.

A ban on hunters' use of lead shot for killing waterfowl was passed in the United States in the early 1990s because birds were being poisoned by ingesting the pieces that fell into waterways and ponds.

But the question of whether to do the same for hunters on land has thrust the eagle, the national symbol of America, into a fresh political battle over gun rights and environmental protection.

On one side is the powerful US gun lobby, which disputes science on lead poisoning and insists that any measures to regulate lead ammunition would spell a ban on hunting in all its forms, infringe on gun rights and raise costs.

On the other is a dogged but weary wildlife protection movement that is pressing the Environmental Protection Agency to take steps to regulate the use of lead ammunition in order to protect birds and humans against lead poisoning.

Both have adopted the bald eagle as a symbol of their efforts, with the bird featuring on the cover of the Center for Biological Diversity's petition to the EPA as well as on the web page of the National Rifle Association.

"This is the last unregulated, widespread distribution of toxic lead into the environment," said Jeff Miller of the Center for Biological Diversity, which is seeking US federal rules to require non-toxic bullets in hunting and shooting sports.

-- Gun-grabbers disguised as nature lovers --

"We know it is getting into the food chain. We also know that humans are eating it and there is no safe level of lead in the human body so it is most certainly a human health issue too."

Miller said 150 groups in 40 states now support the petition, including hunters, scientists, American Indians, conservationists and veterinarians.

The EPA turned down a similar request for a ban on lead bullets in 2010, saying it did not have the authority to regulate ammunition. However, environmental advocates say the EPA does have the right to regulate components of ammunition.

More than a dozen countries in Europe have banned lead ammunition for hunting waterfowl and Norway, the Netherlands, Denmark and Sweden are among a handful of countries that have totally banned lead bullets.

Germany, Japan and Belgium have passed limited restrictions on their use.

The Peregrine Fund, a nonprofit group in Idaho, has posted online a host of peer-reviewed studies on the effects of lead on wildlife, with some figures showing as many as 10-15 percent of young eagles die each year from lead poisoning.

While the bald eagle is no longer a threatened population in the United States, it and other birds that scavenge like the endangered California Condors, vultures, herons and golden eagles, are among the species most at risk.

One study by the Minnesota Department of Natural Resources showed how a lead ballistic tip bullet could fragment into an average of 141 pieces per carcass, reaching as far as 14 inches from the wound (35 cm), indicating a danger for humans who eat meat killed with lead bullets, too.

Pro-gun groups like the National Shooting and Sports Foundation say there is no "sound science" to support a ban.

"If wildlife management decisions become about preventing harm to individuals within a species and not about managing a species itself, then you have essentially made the argument to ban hunting," said Lawrence Keane, senior vice president of the NSSF.

The NRA has urged Congress to "step in and ensure this restriction never happens," asserting that the effort is being headed by "gun-grabbers... disguised as nature lovers."

A House subcommittee in late February approved a bill that would prevent the EPA from taking action on the CBD's petition, and some senators with ties to sportsmen's groups are considering the same.

Rick Watson of the Peregrine Fund said switching to copper bullets costs the same as buying a box of premium lead ammunition, about $45 a box, while the cheapest lead ammo can be had for $15.

"Hunters historically and traditionally have been some of the best conservationists this country has had. And given accurate facts we believe the vast majority will choose to use lead-free ammunition because it protects the wildlife they so enjoy," he said.

Matt Miller, an outdoor writer and hunter, said he switched to copper bullets years ago after learning of the dangers of lead, and is pleased with the results.

"It has not increased the cost of my hunting. The bottom line is if you know your rifle and you shoot it well, a big game hunting trip costs you one bullet."

Burwell, who has been treating her eagle for three weeks and is ready to release her into the wild on Saturday, said she is not optimistic that the EPA will act.

"With the NRA pushing to prevent any type of regulation, the word on the wildlife side is it will never happen," she said.

"It depends on who has the most money. Doesn't it seem sometimes that that is who wins these things?"