In what animal welfare advocates are describing as a “historic advance,” Burger King, the world’s second-largest hamburger chain, said yesterday that it would begin buying eggs and pork from suppliers that did not confine their animals in cages and crates.

The company said that it would also favor suppliers of chickens that use gas, or “controlled-atmospheric stunning,” rather than electric shocks to knock birds unconscious before slaughter. It is considered a more humane method, though only a handful of slaughterhouses use it.

The goal for the next few months, Burger King said is for 2 percent of its eggs to be “cage free,” and for 10 percent of its pork to come from farms that allow sows to move around inside pens, rather than being confined to crates. The company said those percentages would rise as more farmers shift to these methods and more competitively priced supplies become available.

The cage-free eggs and crate-free pork will cost more, although it is not clear how much because Burger King is still negotiating prices, Steven Grover, vice president for food safety, quality assurance and regulatory compliance, said. Prices of food at the chain’s restaurants will not be increased as a result.

While Burger King’s initial goals may be modest, food marketing experts and animal welfare advocates said yesterday that the shift would put pressure on other restaurant and food companies to adopt similar practices.

“I think the whole area of social responsibility, social consciousness, is becoming much more important to the consumer,” said Bob Goldin, executive vice president of Technomic, a food industry research and consulting firm. “I think that the industry is going to see that it’s an increasing imperative to get on that bandwagon.”

Wayne Pacelle, president and chief executive of the Humane Society of the United States, said Burger King’s initiatives put it ahead of its competitors in terms of animal welfare.

“That’s an important trigger for reform throughout the entire industry,” Mr. Pacelle said.

Burger King’s announcement is the latest success for animal welfare advocates, who were once dismissed as fringe groups, but are increasingly gaining mainstream victories.

Last week, the celebrity chef Wolfgang Puck announced that the meat and eggs he used would come from animals raised under strict animal welfare codes.

And in January, the world’s largest pork processor, Smithfield Foods, said it would phase out confinement of pigs in metal crates over the next decade.

Some city and state governments have banned restaurants from serving foie gras and have prohibited farmers from confining veal calves and pigs in crates.

Temple Grandin, an animal science professor at Colorado State University, said Smithfield’s decision to abandon crates for pregnant sows had roiled the pork industry. That decision was brought about in part by questions from big customers like McDonald’s, the world’s largest hamburger chain, about its confinement practices.

“When the big boys move, it makes the entire industry move,” said Ms. Grandin, who serves on the animal welfare task forces for several food companies, including McDonald’s and Burger King.

Burger King’s decision is somewhat at odds with the rebellious, politically incorrect image it has cultivated in recent years.

Its commercials deride “chick food” and encourage a more-is-more approach to eating with its turbo-strength coffee, its enormous omelet sandwich, and a triple Whopper with cheese.

Burger King executives said the move was driven by their desire to stay ahead of consumer trends and to encourage farmers to move into more humane egg and meat production.

“We want to be doing things long before they become a concern for consumers,” Mr. Grover said. “Like a hockey player, we want to be there before the puck gets there.”

He said the company would not use the animal welfare initiatives in its marketing. “I don’t think it’s something that goes to our core business,” Mr. Grover said.

Beef cows were not included in the new animal welfare guidelines because, unlike most laying hens and pigs, they continue to be raised outdoors. Burger King already has animal welfare standards for cow slaughter, he said.

The changes were made after discussions with the Humane Society and People for the Ethical Treatment of Animals, known as PETA.

PETA, in particular, has started a series of high-profile campaigns to pressure fast-food companies to change their animal welfare practices, including a “Murder King” campaign that ended in 2001 when Burger King agreed to improve its animal welfare standards to include, among other things, periodic animal welfare audits.

Since that time, PETA officials said they had met periodically with Burger King officials to encourage them to adopt tougher standards. About a year ago, the Humane Society began its own efforts to encourage Burger King to improve its farm animal standards.

Mr. Grover said his company listened to suggestions from both groups, but ultimately relied on the advice of its animal welfare advisory board, which was created about six years ago and includes academics, an animal welfare advocate, an executive of Tyson Foods and Burger King officials.

“Where we think we can support what our animal advisers think is right, we do it,” Mr. Grover said.

The changes apply to Burger King suppliers in North America and Canada, where the chain purchases more than 40 million pounds of eggs a year and 35 million pounds of pork, he said.

A reason that such a small percentage of purchases will meet the new guidelines is a lack of supply, Mr. Grover said.

Burger King plans to more than double its cage-free purchases by the end of this year, to 5 percent of the total, and will also double its purchases of pork from producers who do not use crates, to 20 percent.

Most laying hens in the United States are raised in “battery cages,” which are usually stacked on top of each other three to four cages high. Sows, during their pregnancies, are often kept in gestation crates, which are 24 inches across and 7 feet long.

Matt Prescott, PETA’s manager for factory farm campaigns, argued that both confinement systems were filthy and cruel because the animals could barely move and were prone to injury and psychological stress.

Under Burger King’s initiative, laying hens would be raised in buildings where they would be able to wander around. Similarly, sows would be raised indoors, most likely in pens where they would be able to move freely.

“This is not free range, but simply having some room to move around inside a controlled environment,” Mr. Grover said.

While converting barns for crate-free sows is relatively simple, Ms. Grandin said it was much more difficult and expensive to raise cage-free hens because not nearly as many birds fit in one building.

Burger King officials say they hope that by promoting controlled-atmosphere stunning, more slaughterhouses will adopt the technology. Currently, there are only a few in the United States using the technique, and most of them process turkeys.