Animal activists have shuddered for years at how supermarket chickens meet their end. Across the industry, they are hung upside down, often breaking their bones, bruising their legs and causing hemorrhages. Workers then cut their throats to kill them, before they're plunked in boiling water.

Now, America's third-largest chicken producer is introducing changes that animal rights groups are championing as a major step forward in animal welfare.

Perdue, which processes nearly 700 million chickens every year at nearly 3,000 farms, announced Monday that it will begin killing chickens using carbon dioxide or argon gas, so they can avoid being hung upside down. With an estimated revenue of $6 billion, according to IBISWorld, they are the first major company to announce that it will use this procedure, which is called controlled-atmosphere stunning.

The new guidelines also said Perdue will expose chickens to more natural light, boost their activity levels and reduce the expectations for how fast chickens must grow. Today, chickens are bred to be so large and produce so much meat that their weight can crush their limbs and immobilize them, said Leah Garces, U.S. director for Compassion in World Farming.

Compassion in World Farming, Mercy for Animals and the Humane Society of the United States worked alongside Perdue for the initiative, culling ideas from smaller organic companies that Perdue had acquired. They follow a rise in consumer demand for better slaughterhouse practices.

"We are seeing from our consumers more questions that are being raised about where their food comes from," Perdue spokeswoman Julie DeYoung said.

Brett Hundley, agribusiness analyst for BB&T Capital Markets, said the initiative could increase costs by 5 to 15 percent, but it appeals to consumers who are increasingly worried about animal welfare.

"A lot of people have been focused on what this could add to your cost profile," Hundley said. "But at the same time, it allows you, at the retail level, to market some of your packaging with the humanely raised assertion. A move like this that goes above and beyond and caters to what consumers care about today — it ideally allows both the companies and customers to differentiate and charge higher prices for their product."

Hundley said Tyson Foods, Pilgrim's Pride and other major poultry companies will "certainly" look into some of Perdue's new intiatives, but it may be years until they adopt them. "They truly care about animal-welfare, social issues, but they also care about producing chickens at the lowest cost for consumers."

Ron Kean, poultry production and management specialist at the University of Wisconsin, also foresees the issue of increased costs. "I think that it will add costs and be less efficient, so they will have to market that claim for a niche market."

Perdue will probably be more appealing to a number of restaurants, retailers and individual consumers, Hundley said. KFC Canada said it will source exclusively birds killed by controlled-atmosphere stunning by the end of the year, and Wendy's, Popeye's Louisiana Kitchen, Chipotle Mexican Grill, Whole Foods Market, Bon Appetit Management and other major restaurants source, though not exclusively, those birds. People for the Ethical Treatment of Animals has advocated for the practice since 2002.

An energy efficient lighting system designed specifically for poultry farms emits a light spectrum adjusted for a chicken's retinas. (Reuters)

This year, Whole Foods announced it would shift to exclusively selling slower-growing chickens by 2024.

For a leading poultry-processing company, it's the "first real effort" to improve chickens' lives, Hundley said. U.S. consumers today care "a lot more about animal-welfare issues than they did 10 or 15 years ago," he said.

It would not be the first time Perdue incited industry-wide change. Perdue was the biggest company to begin limiting antibiotics in 2002. It ceased growth antibiotics in 2007 and then totally eliminated them from their hatcheries by 2014.

"It's safe to assume that the rest of the market chases" Perdue, Garces said. The company is "leading the market in innovative problem solving."

Last year, Tyson Foods, which is the largest chicken-processing company in the United States, said it would eliminate human antibiotics. Pilgrim's Pride, which trails Tyson in chicken production, announced that it would cut back on antibiotics the same month as Tyson.

Still, it will take time for slower-growth chickens killed by controlled-atmosphere stunning and sunlight-bathed coops to be the industry norm. The poultry industry was sluggish in rolling back on antibiotics. The National Chicken Council, a poultry trade group, said as late as 2007 that antibiotics were not proven to be harmful to humans. It did not support Food and Drug Administration bans on antibiotics in 2012, but last year the group said it approved phasing out human antibiotics in chickens.

Eliminating antibiotics makes chicken production pricier, but it also reduces health risk for the people who eat it. Although it's legal in the United States, administering certain antibiotics on otherwise healthy animals is banned in the European Union, Canada and other countries.

In response to whether they will implement policies to limit fast-growing chickens and traditional slaughtering methods, Tyson spokesman Worth Sparkman said the company has ongoing efforts to address animal well-being, such as third-party audits.

"We work hard to be better in everything we do, including animal well-being, and we’re making progress," Sparkman wrote in an e-mail.

Tom Super, senior vice president of communications for the National Chicken Council, said controlled-atmosphere stunning was no more effective at humanely rendering chickens unconscious than the traditional method of low-voltage electrical stunning. Kean, the University of Wisconsin specialist, said that controlled-atmosphere stunning is easier on workers, but it is only negligibly nicer to chickens. "Mostly, it sounds nicer."

But animal rights activists and conscious poultry companies think that it's more than that. "Birds don’t like being hung upside down,” Temple Grandin, a professor of animal science at Colorado State University, said in 2010 to the New York Times. “They get really stressed out by that.”

The policy is not yet flawless, Garces, of Compassion in World Farming, said. She said Perdue should put a timeline on its actions. For instance, it has pledged only to implement controlled-atmosphere stunning at two facilities so far, "followed by gradual implementation at all our harvest facilities," according to a press release. Sunlight will be in only a fraction of slaughterhouses. "We're calling this a good first step."