Some claim that hens’ stress levels vary little between cage, barn and free-range housing systems and that it is animal husbandry practices which have the greatest influence on hen welfare. These claims are often based on research using corticosterone levels (a stress hormone) in the egg albumen (the clear liquid or egg white contained within the egg). Previous studies have suggested there is a positive correlation between the concentration of corticosterone in blood plasma and in egg albumen . However, more recent research has found that because there is so little corticosterone in the egg albumen it can not be considered a reliable indicator of bird welfare [2, 3]. There is a need for further research on the stress physiology of hens in relation to egg-laying behaviour to identify objective and accurate measures of stress and bird welfare.
Any measure used to assess stress and bird welfare should not be considered in isolation of other measurements or without context. In order to make valid comparisons between different housing systems, a variety of welfare and animal-based measurements are required. Chronic housing stress is better examined using several physiological, behavioural and production measures. Evaluating hen welfare should use scientific evidence and consider the three scientific frameworks for assessing animal welfare: biological function, mental state, and natural living. There is a need for animal welfare to be considered not only in terms of the absence of negative states, but also the experience of positive states.
All layer hen housing systems have advantages and disadvantages. The risks to welfare in barn and free-range housing systems can be addressed through good management and system design. However, poor hen welfare is inherent to battery cages regardless of how they are managed. Battery cages do not allow hens to fulfil their behavioural needs for nesting, perching, foraging and dustbathing. The severe space restriction also leads to hens suffering from weak muscles and bones, disuse osteoporosis, and a high rate of fractures during depopulation which occurs at the end of their lives.
 Downing JA, Bryden WL (2008) Determination of corticosterone concentrations in egg albumen: A non-invasive indicator of stress in laying hens. Physiology and Behaviour 95:381–387.
 Engel J, Widowski T, Tilbrook A, and Hemsworth P (2011) Further investigation of non-invasive measures of stress in laying hens. Australian Poultry Science Symposium 126-129.
 Caulfield MP, Padula MP (2020) HPLC MS-MS analysis shows measurement of corticosterone in egg albumen is not a valid indicator of chicken welfare. Animals 10, 821.