There are several welfare concerns regarding the battery cage system of housing and husbandry. These are presented below in the approximate chronological order they would influence the hens.
Chick culling Due to modern
selective breeding, laying hen
strains are different from those of meat production strains. Male birds of the laying strains do not lay eggs and are unsuitable for meat production, therefore, they are culled soon after
being sexed, often on the day of hatching. Methods of culling include
cervical dislocation,
asphyxiation by
carbon dioxide and maceration using a high speed grinder. Animal rights groups have used videos of live chicks being placed into macerators as evidence of cruelty in the egg production industry. Maceration, together with cervical dislocation and asphyxiation by carbon dioxide, are all considered acceptable methods of euthanasia by the
American Veterinary Medical Association. Consumers may also be appalled simply by the death of animals that are not subsequently eaten.
Beak-trimming To reduce the harmful effects of
feather pecking,
cannibalism and
vent pecking, most chicks eventually going into battery cages are beak-trimmed. This is often performed on the first day after hatching, simultaneously with sexing and receiving vaccinations. Beak-trimming is a procedure considered by many scientists to cause acute pain and distress with possible chronic pain; it is practised on chicks for all types of housing systems, not only battery cages. In the US, the current recommendation by the United Egg Producers is per bird. The space available to each hen in a battery cage has often been described as less than the size of a sheet of
A4 paper (). Other people have commented that a typical cage is about the size of a filing cabinet drawer and holds eight to ten hens. Behavioural studies showed that when turning, hens used , when stretching wings , when wing flapping , when feather ruffling , when preening , and when ground scratching 540 to 1005 cm2. A space allowance of would prevent hens in battery cages from performing these behaviours without touching another hen.
Animal welfare scientists have been critical of battery cages because of these space restrictions and it is widely considered that hens suffer boredom and frustration when unable to perform these behaviours. Spatial restriction can lead to a wide range of
abnormal behaviours, some of which are injurious to the hens or their cagemates.
Light manipulation To reduce the harmful effects of feather pecking,
cannibalism and vent-pecking, hens in battery cages (and other housing systems) are often kept at low light intensities (e.g. less than ten lux). Low light intensities may be associated with welfare costs to the hens as they prefer to eat in brightly lit environments and prefer brightly lit areas for active behaviour but dim (less than ten lux) for inactive behaviour. Dimming the lights can also cause problems when the intensity is then abruptly increased temporarily to inspect the hens; this has been associated as a risk factor of increased feather pecking and the birds can become frightened resulting in panic-type ("hysteria") reactions which can increase the risk of injury. Being indoors, hens in battery cages do not see sunlight. While there is no scientific evidence for this being a welfare problem, some animal advocates indicate it is a concern. Furnished cages and some other non-cage indoor systems would also prevent hens seeing natural light throughout their lives.
Osteoporosis Several studies have indicated that toward the end of the laying phase (approximately 72 weeks of age), a combination of high calcium demand for egg production and a lack of exercise can lead to
osteoporosis. This can occur in all housing systems for egg laying hens, but is particularly prevalent in battery cage systems where it has sometimes been called 'cage layer osteoporosis'. Osteoporosis leads to the skeleton becoming fragile and an increased risk of bone breakage, particularly in the legs and
keel bone. Fractures may occur whilst the hens are in the cage and these are usually discovered at depopulation as old, healed breaks, or they might be fresh breaks which occurred during the process of depopulation. One study showed that 24.6 percent of hens from battery cages had recent keel fractures whereas hens in furnished cages, barn and
free range had 3.6 percent, 1.2 percent and 1.3 percent respectively. However, hens from battery cages experienced fewer old breaks (17.7%) compared to hens in barn (69.1%), free-range (59.8%) and furnished cages (31.7%).
Forced moulting Flocks are sometimes force moulted, rather than being slaughtered, to reinvigorate egg-laying. This involves complete withdrawal of food (and sometimes water) for seven to fourteen days or sufficiently long to cause a body weight loss of 25 to 35 percent. This stimulates the hen to lose her feathers, but also reinvigorates egg-production. Some flocks may be force moulted several times. In 2003, more than 75 percent of all flocks were moulted in the US. This temporary starving of the hens is seen as inhumane and is the main point of objection by critics and opponents of the practice. The alternative most often employed is to slaughter the hens instead of moulting them. == Improving welfare for egg-producing hens ==