Pasture fed chickens are grass fed chickens, not to be confused with free-range chickens. the term grass fed is commonly used for cows for their natural diet is mostly grass and other weeds. similarily, chicken are also fed on grass along with vegetables, bugs, weeds, etc.
A free range chicken does not necessarily mean that it is grass fed. Grass fed chickens are those birds which are fed a pasture diet than a grain based diet. Pasture fed chickens forage for grass, seeds and a healthy portion of of insects. Chickens raised on a pasture are recommended by animal welfare groups and environmental organizations.
Grass fed chickens contain a lesser percentage of saturated fat and have a higher vitamin A content. They also contain the good fatty acids in a higher percentage.