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One frequent criticism of the battery cage is that existing cages are too small. The present study used operant conditioning techniques to assess to what extent adult domestic hens were prepared to "work" (key peck) in order to increase the size of their cages. Twenty-seven groups of 4 hens were used. Each group of 4 birds was housed in a cage which had a minimum surface area of 1600 cm2 and a maximum surface area of 6100 cm2. Operant key pecking allowed the birds to increase, and maintain, the size of their cage at any value between the minimum and the maximum. The results obtained indicated that groups of 4 birds preferentially maintain a cage surface area of 2500 cm2 (i.e. 625 cm2/bird). Furthermore birds appeared to avoid a cage surface area of 4300 cm2 (i.e. 1075 cm2/bird). It is suggested that a cage size of 2500 cm2 may be the optimum for groups of 4 hens and that, at least in small groups of laying hens, the relationship between cage size and welfare is not linear.