Cage-free eggs, once a niche product for ethically minded (and well-off) shoppers, are suddenly a hot commodity with an unlikely customer: Big Food. Sonic and Burger King are the latest to join a slate of companies promising to ditch eggs produced by caged hens.

They follow an unlikely trailblazer: McDonald’s, which announced in September that it would go cage-free by the end of 2025. That decision unleashed a “tidal wave of commitments,” says Paul Shapiro, vice president of farm animal protection at the Humane Society of the United States. The list now includes most major American fast-food chains, retailers including Target and Walmart, and food service providers, like Aramark and Sodexo.

Although the number of cage-free birds increased 37 percent last year, they remain less than 10 percent of the nation’s 277 million hens, according to the US Department of Agriculture. Now large egg producers are scrambling to catch up by investing in new cage-free facilities—a swift about-face for an industry that once vehemently fought efforts to eliminate the cramped, paper-sized “battery cages” in which the vast majority of hens spend their lives. In 2008, when California voted on Proposition 2, a measure that mandated that hens should be able to fully spread their wings “without touching the side of an enclosure or other egg-laying hens,” United Egg Producers, the industry’s primary trade group, spent $10 million in a failed effort to defeat the initiative. But this October, UEP President Chad Gregory told Politico that the group wouldn’t put up a fight in Massachusetts, where a measure modeled after California’s will be on the ballot in November.

Most companies, including McDonald’s, have given egg producers up to a decade to change how they house their hens. As Wired charts in detail, the industry is choosing to gradually phase out, rather than dismantle, a production system that’s been designed since the 1950s to provide maximum efficiency. Today, Americans demand 6 billion to 7 billion eggs each month, and they expect every dozen to come relatively cheap.

That means that while cage-free is often portrayed as a nostalgic return to pre-mechanized farming, the newest egg facilities are not like your grandfather’s bucolic little chicken farm. At nonorganic farms, where outdoor access isn’t required, large egg producers are primarily building multitiered aviaries—stacked arrangements in which thousands, if not tens of thousands of birds roam throughout the barn, hopping from level to level. “There are birds by your feet, your knees, your shoulders—cities of birds,” explains Shapiro.

Giving hens the simple ability to move around prevents many of the worst health problems associated with battery cages, Shapiro says, by strengthening brittle bones and allowing them to act on their natural instincts to roost and forage.