Perdue, the country’s fourth largest poultry producer, won plaudits this week when it announced a suite of reforms to its chicken farms: windows, sunlight, access to the outdoors. Most notable, according to one press release, will be Perdue’s bid to replace the traditional way of slaughtering chickens with “controlled atmosphere stunning”—a turn of phrase so bland it can only be deliberate. Because they are, after all, still talking about killing chickens.

Now the current way of killing chickens sounds undeniably grisly. Shackled upside down by their feet, a line of chickens gets stunned in a bath of electrified water before a rotating blade cuts their throats. Controlled atmosphere stunning, on the other hand, uses gas to knock the birds out before they die of oxygen deprivation or later bleed-out. The word I heard several times—from Perdue as well as the Humane Society of the United States—was “gentle.”

The thing is that the stunning technique may not always look gentle. Controlled atmosphere stunning may sound like precise, technical jargon, but it refers to a collection of practices with gas—sometimes it means "irreversibly stunning" (aka killing) the chicken and sometimes it means reversibly stunning the chicken long enough for it to be killed with a cut to the throat while unconscious. Different machine run on different combinations of carbon dioxide, argon, or nitrogen. And depending on the gas and its concentration, birds may convulse wildly, even break their wings. As obviously terrible as this sound, it’s hard to interpret: It could be a conscious animal fighting for its life or it could be an unconscious animal spasming out of control—a bizarre physiological response can also happen to humans going under anesthesia.

Animal welfare is tricky when you get into the details; it means, essentially, a bunch of humans sitting around considering what it feels like to be a chicken. To understand, you have to get inside the head of a chicken—maybe literally. And if you’re a scientist whose job is to study animal welfare, then you’re putting EEGs on chicken to measure their brain waves.

Inside a Bird Brain

Researchers in Europe have been putting EEGs on chickens since the 80s, originally to study stunning with electrified water. With the right current, EEG activity spikes, and the chicken is basically unconscious. But the chickens still have to be shackled upside down while awake, and some animals, especially the smaller ones, may miss the head dip and go to the blade fully conscious.

Over the past decades, European researchers have developed controlled atmosphere stunning using gas. But which gas? Nitrogen or argon, which are inert gases, are two possibilities. Chickens, like humans, regulate breathing primarily by sensing the buildup of carbon dioxide rather than the lack of oxygen. (What makes holding your breath painful is the carbon dioxide.) Replacing oxygen with nitrogen or argon could be largely painless. The problem is the convulsing.

When Dorothy McKeegan, a veterinarian at University of Glasgow, used EEGs to study chicken's brains though, they didn’t seem to start convulsing until their EEGs already had a pattern linked to unconsciousness, like when a person goes under anesthesia. But the vigorous flapping is still a big problem if you don’t want to sell bruised and broken chicken wings.

A second method is replacing oxygen with carbon dioxide. Carbon dioxide is cheap (unlike argon), and you don’t have to reduce the oxygen to very, very low levels (unlike with argon or nitrogen) to get an effect. But carbon dioxide, to which chickens are very sensitive, causes “air hunger.” “You can see the birds make these big breaths,” says McKeegan. “There’s been a lot of debate over how nasty that is for the birds, how much it causes a welfare problem.” EEGs can tell scientists if the bird is conscious, but it can’t tell scientists if a chicken is in pain.

A more humane method appears—and again this is what appears—to be gradually ratcheting up the level of carbon dioxide, so the chickens get sleepy and pass out before they actually die of oxygen deprivation. This is the method that Perdue is looking to implement in its 10 chicken processing plants.

Perdue currently has one turkey plant in Washington, Indiana, that uses controlled atmosphere stunning, and it’s looking to convert its first chicken processing plant by the end of 2017. “We have a lot to learn with the first one,” says Bruce Stewart-Brown, senior vice president for food safety and quality at Perdue. The company is still deciding exactly which plant to convert first and which CAS company’s equipment to use.

For Stewart-Brown, the key is no longer having to unload and shackle conscious chickens—one of the worst jobs in the slaughterhouse. The area is dark (to avoid scaring the birds with bright lights) and loud (because the chickens are nevertheless scared). “It’s typically a rough area to keep people and to see,” says Stewart-Brown. With CAS, the gas can be pumped directly into the truck transporting chickens. “This turns that area of the plant into just a way better place.” Josh Balk, director of food policy at the Humane Society of the United States, echoed the idea. “What Perdue is doing is good for animals and workers,” he says. And the benefit to workers, at least, will be far easier to measure.