Although, for example, it has long been known that the stocking density of fewer birds per cage leads to a greater production per bird, it is nonetheless more economically efficient to put a greater number of birds into each cage, accepting lower productivity per bird but greater productivity per cage. The most obvious problem is lack of exercise and natural movement. Under free-range conditions, hens walk a great deal. Wing flapping, which is common in free-range animals, is also prevented in cages. Comfort behavior is likewise truncated, as is leg stretching and preening.
Lack of exercise has serious effects on bones and muscles.
Caged birds have greater incidence of lameness, bone brittleness, osteoporosis, and muscle weakness than uncaged ones. Most significant, battery hens have a much higher incidence of broken bones than animals free to move. House and cage design must facilitate optimal daily care and inspection of the birds. Cages should be designed and maintained so as to avoid injury to the birds. Cage, feeder, and waterer construction should take into account proven advantages for bird comfort and health, and facilitate the safe removal of birds.