Cage layer fatigue was first noticed after laying hens began to be housed in cages in the mid-20th century. Hens producing eggs at a high rate were most susceptible to the disease. Early research revealed that cage layer fatigue was associated with osteoporosis and bone brittleness. Severe osteoporosis leads to spontaneous bone fractures commonly in the costochondral junctions of the ribs, the keel, and the thoracic vertebrae. Vertebral fracture may damage the spinal cord and cause paralysis. Osteoporosis appears to be inevitable in highly productive caged laying hens. The condition can be made worse by metabolic deficiency of calcium, phosphorus, or vitamin D. Hens in housing systems that promote physical activity tend to have less osteoporosis and rarely manifest cage layer fatigue. Genetic selection may produce laying hens that are less prone to bone weakness. The welfare implications of osteoporosis stem from pain, debility, and mortality associated with bone fracture. The chicken has well-developed neural and psychological systems specialized to respond to pain associated with trauma and inflammation. Although studies on the chicken have not focused on pain due to bone fracture, physiological and behavioral similarities to other species allow inference that a hen experiences both acute and chronic pain from bone fracture. There is little information on osteoporosis in commercial caged layer flocks, however, evidence suggests that it may be widespread and severe. If true, most caged laying hens suffer osteoporosis-related bone fracture during the first laying cycle. Osteoporosis also makes bone breakage a serious problem during catching and transport of hens prior to slaughter. Estimates of mortality due to osteoporosis in commercial caged layer flocks are few, but range up to a third of total mortality. Many of these deaths would be lingering and attended by emaciation and possibly pain. Osteoporosis-related bone breakage during processing has reduced the marketability of spent caged laying hens, contributing to the need to develop humane on-farm killing methods to support alternative means of spent hen disposition. Overall, the evidence indicates that cage layer osteoporosis is a serious animal welfare problem. A determined effort must be made to make the laying hen no longer susceptible to the harmful effects of excessive bone loss.

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Observations of the behavioral reactions of laying hens and broilers to different gas stunning atmospheres were made. Sixty Hy-Line W-36 hens and 60 market-weight commercial broilers were placed individually into a plexiglass gas stunning unit and exposed to one of six gas atmospheres: air, concentrations of 30, 45, or 60% CO2 in air, a mixture of 70% argon and 30% CO2, and 100% argon. Video records were made during each test, which lasted until the subject became unconscious or for 2 min in the air treatment. Behavior in the 100% argon atmosphere resembled that in air, until birds became impaired by anoxia. All treatments involving CO2, including 70% argon/30% CO2, caused deep breathing and head shaking. The concentration of CO2 in air in the range tested did not affect the tendency to perform different actions, except that birds in 60% CO2 were more likely to exhibit a convulsive flip at the point of collapse. Chickens in 70% argon/30% CO2 tended to demonstrate less sedation and performed more sudden efforts to regain balance during tests than did chickens in CO2 mixtures in air and were more likely to perform a convulsive flip. Deep breathing and head shaking have been suggested as being indicative of respiratory distress and aversive reaction to CO2. The data in this study are consistent with the possibility that head shaking is an alerting response functioning to promote arousal in the face of reduced sensibility during exposure to CO2-enriched atmospheres. Nonetheless, if the view is correct that deep breathing and head shaking indicate distress, the 70% argon/30% CO2 gas mixture was at least as distressing as even 60% CO2 in air. The relative prevalence of sudden efforts to regain balance in 70% argon/30% CO2 suggest that this gas mixture might cause even more distress than up to 60% CO2 in air.

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