The campaign to force America's farmers to change the way they handle their animals celebrated a victory this week.
McDonald's USA announced that in the near future, it will no longer buy eggs from chickens that live in cages.
Those cages are still the industry standard, and 90 percent of America's eggs come from chickens that live in them.
When egg farmers adopted them, decades ago, they thought it was progress. Chicken manure didn't pile up on the floor anymore. Chickens weren't walking around in it. "The eggs were super clean. The feed, the water, everything in these houses became super clean," says Chad Gregory, president of the United Egg Producers, the main industry association.
But animal welfare advocates have been fighting the cages, with some notable successes.
California now demands that all egg-laying chickens at least have enough room to stretch their wings and turn around.
It's possible to meet that requirement with "enriched colony cages" that give chickens more room and nests to lay their eggs. In fact, Gregory says that's the most popular alternative to traditional cages among farmers.
"They would all say that enriched colony cages are the best for the environment, for the cost, for animal welfare, for food safety," he says. This is backed by some industry-funded research.
Consumers, however, find the "cage-free" label more attractive. As the name indicates, cage-free eggs come from chickens that are allowed to roam freely around the chicken house.
This week, McDonald's joined the movement. The maker of Egg McMuffins announced that within 10 years, all of its American and Canadian egg suppliers will be cage-free. And it may need more of those Egg McMuffins: The company recently announced plans to serve breakfast all day long.
Chad Gregory says that farmers who've made the switch are making a pleasant discovery. "They're finding out that those cage-free systems aren't as scary as they once feared," he says.
New cage-free structures allow manure to be removed more easily than in old-style houses, for one thing.
So farmers are hedging their bets, building both enriched cages and cage-free houses.
Old-style cages, meanwhile, appear to be on their way out, although they may not disappear completely for many years.