Modern laying hens suffer epidemic rates of osteoporosis and bone fractures driven by the physiological demands of sustained high egg production.
Keel bone fractures represent one of the clearest examples of a welfare problem created by selective breeding for production traits without regard to the animal experiencing the consequences. The physiology of sustained high egg production imposes calcium demands that progressively weaken bones, and the transition from cages to cage-free systems has in some cases worsened fracture rates through increased fall injury. The result is a welfare problem that neither housing system type alone can solve: cage-free hens have more freedom but more fractures; caged hens have fewer fractures but severe restriction. Genuine welfare improvement requires addressing the root cause through genetic selection that balances production with bone health, alongside housing and management improvements that reduce fall and collision risk.