The Vermont ice cream maker Ben and Jerry's got plenty of attention last September when it became the first major food manufacturer to announce it would use only cage-free eggs that have been certified humane by an inspecting organization. But the company says it will need four years to complete the switch. "It's not easy to find all the eggs you're looking for," said Rob Michalak, a spokesman for Ben and Jerry's. "The marketplace is one where the supply needs to increase with the demand."

The eggs can cost an extra 60 cents a dozen on the wholesale market. But most chicken farmers are not ripping out cages and retrofitting their barns. They question whether the birds are really better off, saying that keeping thousands of hens in tight quarters on the floor of a building can lead to hunger, disease and cannibalism. They also say that converting requires time, money and faith that the spike in demand is not just a fad.

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Growing consumer concern with farm animal welfare and interest in local and sustainable agriculture have driven some of the popularity, but campaigns by animal rights activists have had a lot to do with it. The Humane Society of the United States began a campaign against battery cages in 2005, pressuring egg producers to improve conditions and companies to change their policies. Last week, the group took on Wendy's with a series of print and radio advertisements urging the company to follow Burger King's lead on eggs. In a battery cage, the area allotted to each chicken is about the size of a laptop computer. Opponents say that in such small spaces, chickens cannot stretch their wings, roost or engage in other natural behaviors. This year, the Humane Society convinced the chef Wolfgang Puck that cage-free chickens make better-tasting eggs. Although the look and taste of an egg are most affected by its age and the chicken's diet, many chefs believe that cage-free eggs are of higher quality. But not all cage-free eggs are equal.

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"While cage-free certainly does not mean cruelty-free, it's a significant step in the right direction," said Paul Shapiro of the Humane Society.