1. Anonym, 1979 a. Intensive production and farm animal welfare. Danish Ministry of Justice. Table 19.
2. Anonym, 1979 b. Ibid.Document no. 31 (Bone fratures in hens).
3. Anonym, 1981. The physiological and ethological needs of egg- layers to perform egglaying behaviour and wing-flapping. Reviews of literature presented by Society for Veterinary Ethology to The Standing Committee of the European Convention for the Protection of Animals Kept for Farming Purposes TPA/E (81) 8.
4. Bareham, J.R. 1972. Effects of cages and semi-intensive deep litter pens on behaviour, adrenal response and production in two strains of laying hens. Br.Vet.J., 128, 153–163.
5. Baumeister, A., Hawkins, W.F. & Cromwell, R.L., 1964. Need states and activity level. Psychol.Bull., 61, 438–453.