Hughes B O, Wood-Gush D G
Anim Behav. 1977 Nov;25(4):1056-62. doi: 10.1016/0003-3472(77)90056-2.
The incidence of agonistic behaviour was recorded over 1 year in laying hens of two strains, housed either in battery cages or in deep-litter pens in groups of either three or six birds. Aggressive head-pecking was more frequent in pens than in cages and in groups of six than in groups of three. Threats were more frequent in pens and in groups of six light-hybrids. Other pecking at the plumage was more common in cages in groups of six and in the light-hybrid than in the medium-hybrid strain. Observations showed that hens occupy a considerable area when engaging in threat displays: this suggests that the physical restraint of crowding may play an important part in limiting this behaviour. Two reasons for the reduction of aggressive head pecking in caged birds were suggested: firstly, inhibition of the subordinate hens' behaviour because they were constantly in the sphere of influence of the dominant bird and secondly, because agonistic encounters may be triggered only by the entry of other birds into an individual's 'personal space' and not by continuous proximity.
