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Cage size
Floor space for battery cages range from 300 cm² per bird and up. EU standards in 2003 called for at least 550 cm² per hen. In the US, the current recommendation by the United Egg Producers is 67 to 86 in² (430 to 560 cm²) per bird.
The space available to battery hens has often been described as less than the size of a piece of paper. A4 sized paper has an area of ≈ 97 in² (625 cm²), while letter sized paper has an area of ≈ 93.5 in² (603 cm²). A typical cage is about the size of a filing cabinet drawer and holds eight to 10 hens. Hens in battery cages do not have room to lie down or stretch their wings.
Animal welfare
The main contentious issue of battery cages relates to the welfare of the hens. Several studies have indicated that a combination of high calcium demand for egg production and a lack of exercise lead to a painful condition known as cage layer osteoporosis, which increases the chances that hens in battery cages will break their bones.
However, it is the behavioural effects of keeping hens in cramped and barren conditions that is the main concern of both animal welfare organisations and scientists studying animal welfare. The Scientific Veterinary Committee of the European Commission stated that "enriched cages and well designed non-cage systems have already been shown to have a number of welfare advantages over battery systems in their present form". Hens in battery cages are said to experience elevated stress and aggression levels, but a 2008 study states that there is no detectable difference in stress levels based on housing.
Battery cages
Animal welfare scientists have been critical of battery cages because they do not provide hens with sufficient space to stand, walk, flap their wings, perch, or make a nest, and it is widely considered that hens suffer through boredom and frustration through being unable to perform these behaviours.
Supporters of battery farming contend that alternative systems such as free range also have welfare problems, such as increases in cannibalism and injurious pecking. A recent review of welfare in battery cages made the point that such welfare issues are problems of management, unlike the issues of behavioural deprivation, which are inherent in a system that keeps hens in such cramped and barren conditions. Free range egg producers can limit or eliminate injurious pecking through such strategies as providing environmental enrichment, feeding mash instead of pellets, keeping roosters in with the hens, and arranging nest boxes so hens are not exposed to each others' rears.
Human health
Research has shown that the risk of salmonella is likely to be higher in intensively produced eggs in comparison to free-range or organic produced eggs.