Burger King today became the largest restaurant chain to put a time frame on their pledge to purchase only cage-free chicken and pork products.

The fast-food giant vowed to use only cage-free eggs by 2017. A similar promise was made concerning pork from sows not raised in gestation stalls.

"These changes by Burger King Corp. will improve life for countless farm animals and encourage other companies to abide by animal welfare principles up and down their supply chain," said Humane Society of the United States president Wayne Pacelle.

Burger King customers purchase millions of pounds of egg and pork products each year, according to the Associated Press. Nine percent of all eggs and 20 percent of all pork products are already cage-free. McDonalds and Wendy's are reportedly making similar plans to uncage their food sources, but have yet to set an exact date.

Even with a timetable in place, the shift to all cage-free comestibles won't come easily: In 2010, only 4% of the 10.5 million egg-producing hens were allowed to roam unfettered.

Still, as the humane society's VP for farm animal protection Paul Shapiro points out, "This is an issue that just four to five months ago was not on the food industry's radar; now it's firmly cemented into the mainstream in a way that I think few people would have imagined."

[photo via AP]