What a difference a year can make. Just ask the egg industry.

Last September, McDonald’s made its historic announcement to stop using eggs from caged chickens, by phasing in 100 percent cage-free eggs for all the two billion eggs it uses at its U.S. and Canadian restaurants.

At that time, virtually no major restaurant or grocery chains had policies to end their use of eggs from caged hens. Yet just 12 months after McDonald’s announcement, essentially every single one now does.

What happened?

OPINION

For years, the animal protection movement had campaigned against the egg industry’s practice of locking birds in cages. While there are essentially no rules for how much space producers must give their birds, each animal typically has less space than an iPad on which to live for more than a year before she is slaughtered. In fact, nine out of ten birds in the U.S. egg industry are currently trapped inside cages so cramped they can’t even spread their wings.

In response to these animal welfare concerns, some states phased in cage-free conditions for their laying hens.

For example, California’s hen protection law took effect in 2015, with Michigan’s similar law set to be fully implemented in 2020.

But in mid-2015, animal advocates led by The Humane Society of the United States announced the latest salvo in the battle over factory farming: a 2016 ballot measure to require that eggs sold in Massachusetts be produced only by cage-free hens. And news of the measure spread like wildfire throughout the egg industry.

Producers — especially those in the Midwest — had thrown money trying to defeat California’s cage-free ballot measure, only to lose in a political landslide: 64-36. So when the Massachusetts ballot measure was announced, many producers acknowledged that they couldn’t win with voters and pledged not to fight in the Bay State, essentially conceding that the entire Commonwealth’s market would be going cage-free.

As public policies to give hens better lives were taking effect, the pressure continued mounting on companies to change their supply chains. Shareholder proposals with major retailers were being filed, whistleblowing exposés at the big egg factories were garnering national headlines, and some big names in food — like Unilever, Burger King, Compass Group, Aramark and Sodexo — were announcing their own cage-free phase-ins.

The writing was starting to appear on the wall, but still nearly all the major egg producers were defending cage confinement. McDonald’s had been committed to studying the issue for three years, and no one knew what conclusion the food behemoth might reach. And in the end, the company did the right thing. Recognizing the difficulty of defending what most Americans consider indefensible, one of the biggest egg buyers in America proclaimed its future to be cage-free.

When a company like McDonald’s speaks, markets move. And indeed, in just the one year since the  announcement, every major restaurant and grocery chain has adopted similar or even faster timelines for cage-free conditions in their egg supply chains. The companies with these policies now number in the hundreds, and include giants like Walmart as well as major egg producers themselves.

Unlike a year ago, the question isn’t if egg companies — overwhelmingly located in the Midwest — will stop forcing birds into cages. It’s just how fast that transition will occur.

Perhaps it won’t be that long before people look back in amazement that there was ever a debate about whether egg producers immobilize their birds. And this past year will be seen clearly as the tipping point in that societal shift.

Paul Shapiro is vice president of farm animal protection for The Humane Society of the United States.

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