CHICAGO — Nine years after calling a truce with McDonald's Corp., People for the Ethical Treatment of Animals says it is going on a new offensive against the fast-food giant, this time over the most humane way to kill a chicken.

Should chickens be knocked unconscious with a jolt of electricity and then have their throats cut, the conventional method of slaughter in this country? Or should they be gassed, a practice used to some extent in Europe?

PETA, known for its in-your-face protest style, claims the latter induces less suffering. So it's taken its cause to the U.S. chicken industry's biggest customers, including McDonald's and KFC Corp.

PETA has been waging war against KFC since 2003 with its "Kentucky Fried Cruelty" campaign, boycotting the firm, staging thousands of protests at KFC restaurants and using shock tactics like dousing company executives in fake blood.

KFC has refused to give in.

"When PETA protests our restaurants, we have to add additional staff because sales actually increase while they're there," the company said in a statement to the Chicago Tribune.

Still, KFC franchisees in Canada, representing about 750 restaurants, agreed last year to begin buying chickens from suppliers that use the gas slaughter method.

Now, PETA is turning its attention to McDonald's.

PETA kicked off a protest against McDonald's at a restaurant in Chicago's River North area yesterday featuring rock star Chrissie Hynde. PETA is largely reinstating the "McCruelty" campaign it last used against McDonald's in 2000.

McDonald's made major animal welfare changes in 2000, changes it insisted had nothing to do with PETA, including auditing slaughterhouses to ensure humane treatment.

PETA is aiming to pressure McDonald's and KFC into convincing U.S. chicken suppliers to rejigger their plants to the gas method of slaughter. McDonald's has the power to "require that changes be made" by its suppliers, said Matt Prescott, PETA's director of corporate affairs.

McDonald's has studied the chicken issue extensively, including conducting its own tests on the gas method of slaughter, said Bob Langert, McDonald's vice president of corporate social responsibility. "It's not conclusive that it's more humane."

About 30 percent of the chicken McDonald's buys in Europe comes from slaughterhouses that use the gas method. But in this country, the technology hardly exists in the chicken industry.

In most U.S. chicken slaughterhouses, birds are plucked from big bins by workers and hung upside down on assembly lines. Their heads are dragged through a brine bath and a shock is administered, which if done properly knocks the bird senseless. A whirring blade then cuts its throat.

In the method advocated by PETA, as well as the Humane Society of the United States, chickens are gassed and then hung on an assembly line after they are dead.

"It causes less suffering than the conventional method, which is archaic and inhumane," said Paul Shapiro, head of the Humane Society's factory farming initiative.

But there is disagreement even within animal welfare circles.

"There is not definite proof either is more humane," said Marie Wheatley, president of the American Humane Association. "Both technologies are acceptable in minimizing pain and suffering."

Temple Grandin, a Colorado State University expert on humane animal handling, said both methods can have problems, though gas is the best bet for the future.

"Chickens don't like being hung upside down. They get stressed out," said Grandin, who is a member of McDonald's Animal Welfare Council, a group of outside experts who offer pro bono advice. Plus, worker abuse of live chickens on the assembly line is a serious problem, she said.

On the other hand, if gas levels aren't administered correctly, a chicken's death can be quite painful. "I have seen them go berserk," Grandin said. And even when gas is administered properly, chickens still gasp and their heads shake, she said.

Still, Grandin said the chicken industry must move toward the gas method, even though it will require costly investment.

"I'd like to see someone in the industry put up a full-scale commercial plant and make it work," she said.