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There have been claims that egg industry-commissioned research has demonstrated hen stress levels vary little between cage, barn and free-range farming systems and that it is animal husbandry practices which have the greatest influence on hen welfare. The unpublished egg industry-commissioned research analysed the stress hormone (corticosterone) in egg albumen (the clear liquid or egg white contained within the egg). The concentrations of stress hormone in eggs did not differ between housing systems but instead varied between individual farms, and suggested that differences in corticosterone levels had to do with how the birds on a particular farm were managed and what challenges (physiological and environmental) they had to cope with. However, there is a need for peer-reviewed, published research in this area, and the combination of measurements of stress physiology with other measurements of welfare.
Studies have assessed the correlation between the concentration of corticosterone in blood plasma and in egg albumen. Downing and Bryden (2008) found that it is positively correlated, and that corticosterone in egg albumen may provide a non-invasive measure of stress. However, Engel et al. (2011) found few correlations between corticosterone concentrations in blood plasma, albumen, yolk, and faeces. The correlation between corticosterone concentrations in plasma and egg albumen needs further validation, and research on the stress physiology of hens in relation to egg-laying behaviour is very limited.
Using non-invasive measures of stress, such as corticosterone in egg albumen, are preferable to more traditional methods such as blood sampling where the handling of the bird will result in a certain level of stress. However, the use of corticosterone in the absence of other data and without being able to link this to individual birds is of limited value. Corticosterone concentrations in eggs are just one measure which may be used to assess stress and welfare, and should not be considered in isolation of other measurements. In order to make valid comparisons between different housing systems, a variety of welfare measurements are required. Chronic housing stress is better examined using several physiological, behaviour and production measures. An example is El-Lethey et al. (2000), who showed a range of effects, including lower stress levels in floor housed hens provided with straw compared to hens not provided with straw.
Hen welfare should be evaluated using scientific evidence on structure and function and also on behaviour, considering 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 free range and barn systems may be addressed by 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 behavioural needs for nesting, perching, foraging and dustbathing. The severe spatial restriction also leads to very weak muscles and bones, disuse osteoporosis, and a high rate of fractures which occur at the end of their lives. The disadvantages of battery cages can only be overcome by fundamentally changing the housing system.