Hemsworth, P. H., Barnett, J. L., Jones, R. B. 1993. Situational factors that influence the level of fear of human by laying hens. Applied Animal Behaviour Science 36, 197-210.

On the basis of the withdrawal responses of birds to an approaching experimenter, fear of humans may decline from moderate levels among birds housed in Tier 1 (bottom) to Tier 3, in both systems and then sharply increase from Tier 3 to Tier 4, in the four-tier system, reaching the highest level recorded.