Effect of group size on aggressive reactions to an introduced conspecific in groups of domestic turkeys (Meleagris gallopavo)

Aggressive encounters and injuries due to head pecking seriously threaten the welfare of domestic turkeys and also result in economic losses for the turkey industry. Animals of different species show an increase in aggression when unfamiliar conspecifics are mixed but a marked drop in aggression occurs as they become familiar with each other. We conducted an experiment to investigate the effect of group size on the frequencies of different types of aggressive interactions performed by 12-week-old broad-breasted turkey toms. We introduced non-group members for 30 min into 12 small (6 birds) and 12 large groups (30 birds; 432 birds in total) and observed the effect of this short term introduction on the subsequent incidence of aggressive behaviour. Half of the introduced animals originated from small groups and half from large ones. Members of the small groups initiated more fights (0.9/0.1) and delivered more aggressive pecks (20.3/5.0) to the introduced bird than members of the large groups. Also, birds originating from small groups initiated more fights (1.4/0.6) and delivered more aggressive pecks (7.4/3.2) than birds from large groups. Group members showed virtually no aggression towards reintroduced members of their own group. We demonstrate that groups of turkey toms react differently to an introduced conspecific depending on their group size and whether the introduced bird is a group member or not. (c) 2005 Elsevier B.V. All rights reserved.