SALISBURY, Md. — The chickens in one pen were, for the most part, doing what they usually do toward the end of their lives on a factory farm: resting on the floor, attacking the feeding pan, getting big fast.

But in the next pen over, smaller, leaner birds of the same age ran around, raising a ruckus as they climbed on haystacks, perched on roosts and gave themselves dirt baths.

“We’re going to have to come up with a sturdier water line,” said Dr. Bruce Stewart-Brown, a veterinarian and senior vice president of Perdue Farms, as he watched two of them swing the tube that supplies water to the pen.

The frisky birds and their more sedentary neighbors here in a barn on the Delmarva Peninsula are part of an experiment that could help change the way Americans eat, and think about, poultry.