The "best" chicken, for the past century, has been one that put on muscle quickly. But according to Muir, a trait that was great for the poultry industry turned out to be not so great for the chickens themselves.
All that weight, accumulated quickly, can overwhelm a young chicken's bones.
"We're having problems with legs," Muir says. "They can't support the weight. We have problems with splayed legs, joint problems. This is a major well-being concern, if the bird can't walk."
Studies have observed these problems in anywhere from 15 to 30 percent of chickens grown for their meat, known as broilers.
Broiler chickens are slaughtered when they're still young, but others are kept around in order to reproduce. They are the so-called breeder flocks — the source of the billions of chickens that people eat.
Because these chickens live longer, Muir says, they have a special problem. "They're so big and heavy, if we let them keep on eating, they couldn't reproduce. So they have to be on a diet, a severe diet, and they're always hungry."
For all those reasons, some animal welfare advocates have been calling on poultry companies to turn back the clock and return to slower-growing breeds of chickens.
Those breeds still do exist. Poultry producers can order them from the same big chicken-breeding companies that created the fast-growing chicken.
There's increasing demand for slower-growing breeds in Europe. In the U.S., they're mainly used by farmers who want to raise their chickens the old-fashioned way, running around outdoors.
Theo Weening, the global meat buyer for Whole Foods Market, says his stores do stock small quantities of such pastured poultry. "Actually, I'm on my way this afternoon to Arkansas, to Crystal Lake Farms, [which] uses a slow-growing chicken," he told The Salt.
Whole Foods recently announced that it wants all of its suppliers, even those raising large numbers of broilers indoors, to shift over to slower-growing breeds of chickens.
The shift will take eight years. Whole Foods and the Global Animal Partnership, an organization that Whole Foods set up to create welfare standards for its suppliers, say that this will apply to 277 million birds annually. That represents about 3 percent of the country's broilers.
The slow-growing bird "is a much better, healthier chicken, and at the same time it's a much [more] flavorful chicken as well," Weening says.
But he admits that this chicken will come with a cost. Slower-growing breeds consume more feed per pound of meat. According to data on the website of the poultry breeding company Aviagen, a slower-growing breed called Rowan Ranger consumes about 25 percent more feed while growing to a weight of 6 pounds than the Ross 308 breed, which the company says is the "world's most popular broiler."
After the Whole Foods announcement, the National Chicken Council, which represents major poultry producers, released a statement disputing the idea that faster-growing chickens are less healthy or are suffering.
The Chicken Council also pointed out the benefits of the industry's push for ever greater efficiency. It has cut the cost of growing chickens, reduced the amount of land required to grow chicken feed, and made chicken the most popular meat in America.