Housing systems for egg-laying hens range from small, pasture-based flocks to large, commercial-scale operations that intensively confine tens of thousands of hens indoors. The overwhelming majority of laying hens used for commercial egg production in the United States are confined in battery cages and provided 432.3 cm2 (67 in2) of space per bird. Cages prevent hens from performing the bulk of their natural behavior, including nesting, perching, dustbathing, scratching, foraging, exercising, running, jumping, flying, stretching, wingflapping, and freely walking. Cages also lead to severe disuse osteoporosis due to lack of exercise. Alternative, cage-free systems allow hens to move freely through their environment and to engage in most of the behavior thwarted by battery-cage confinement. Given their complexity, cage-free systems can be more challenging to manage and may require superior husbandry skills and knowledge. Laying hens must be genetically suited to the alternative housing system to realize its full welfare advantages. Regardless of how a battery-cage confinement system is managed, all caged hens are permanently denied the opportunity to express most of their basic behavior within their natural repertoire. The science is clear that this deprivation represents a serious inherent welfare disadvantage compared to any cage-free production system.
Shields, Sara and Duncan, Ian J.H., "A Comparison of the Welfare of Hens in Battery Cages and Alternative Systems" (2009). IMPACTS ON FARM ANIMALS. 18.