Fraser, A. F. , Broom, D. M. 1990. Farm Animal Behaviour and Welfare - Third Edition. Bailli?re Tindall, London, UK.

Description of the species-typical and abnormal behavior of farm animals. Wool-pulling is generally performed by the most dominant sheep and is directed at subordinates. The sheep which receive most wool-pulling are usually those which are lowest in the social hierchy within a group. The condition is therefore related to social dominance. .. Since this conditions is clearly associated with overcrowding within indoor pens, control of the condition is possible through re.duction in pen densities. It is most frequent at high stocking density and can be eliminated by increasing space per animal. Feather-pecking is a form of anomalous behaviour which is common in poultry. ... The normal exploration and food investigation behaviour of such birds involves pecking, so it is not surprising that in a barren environment they investigate the feathers of other birds in this way. .. Adult birds try to avoid being pecked and often assume a stooped submissive position. In these cases it appears that social dominance dictates the right of one bird to peck at another. .. Feathers which are pecked from other birds may sometimes be eaten [in intensive husbandry systems]. ... Body-pecking and consequent cannibalism can begin when wounds arise due to feathers being pulled out.