Hansen, I., Braastad, B. O., Storbr?ten, J. et al. 1993. Differences in fearfulness indicated by tonic immobility between laying hens in aviaries and in cages. Animal Welfare 2, 105-112.

In the present study the fearfulness, as measured by the tonic immobility response, increased more with age in battery layers than in aviary layers. The more constant and lower fearfulness of birds in the aviaries suggests that with regard to the fear concept these systems offer a higher level of welfare.