Dynamics of aggression in the domestic fowl

We conducted three experiments designed to investigate the effect of group size (GS) on the frequencies of different types of aggressive acts performed by 11-21-week-old White Leghorn pullets. In these experiments, we systematically controlled for bird density, pen size, and previous social experience in groups of different sizes. Aggression was measured in the context of competition for a limited supply of highly attractive food provided in two patches (central and peripheral) in addition to the regular (ad libitum) food supply. The birds were observed in their original groups of 15, 30, 60 and 120 birds (3-4 replicate groups per GS), reared together since hatch, or in temporary sub-groups of the original groups. In the first experiment (space per bird constant, pen size and GS history varying), the number of aggressive pecks delivered per food patch declined as GS increased from 15 to 120 birds. In the second experiment, utilising birds from the groups of 120 only (space per bird varying, pen size and GS history constant), the number of aggressive pecks delivered per patch increased as temporary GS increased from 15 to 60 birds. In the third experiment (space per bird and pen size constant, GS history varying), the number of aggressive pecks delivered per patch was constant among groups of 15 birds obtained from the groups of 15, 30, 60 and 120 birds. In all experiments, the central food patch was exploited sooner than the peripheral one. The actual number of birds at a food patch at a given time was more important in determining the frequency of aggression at the food patch than the number of birds present in an enclosure or previous GS experience. When a high number of individuals was present at a food patch, aggression at that patch was relatively infrequent. The results support the hypothesis that aggression in the domestic fowl is a dynamic process, with decisions about aggressive behaviour being made facultatively according to the relative costs and benefits of different behavioural strategies at a given time and place rather than being fixed obligatorily at a constant level according to the number of birds present in an enclosure and their ability to form dominance relationships. (C) 2002 Elsevier Science B.V. All rights reserved.