Behavioural and adrenocortical responses of domestic chicks to systematic reductions in group size and to sequential disturbance of companions by the experimenter.

Disruption of an animal's social environment often causes distress and the effects of systematic reductions in group size on the behaviour and plasma corticosterone concentrations of domestic chicks were assessed in Experiment 1. Because this procedure also exposed the chicks to repeated disturbance by the experimenter, the behavioural and adrenocortical consequences of sequential capture and replacement of social companions were examined in Experiment 2. Female White Leghorn chicks were housed in groups of eight and remained undisturbed in both experiments until testing began at 14 d of age. Systematic reduction of group size was accompanied by both endocrine and behavioural modifications in Experiment 1. Thus, plasma corticosterone levels rose, feeding, drinking, preening and pecking at the environment gradually ceased whereas defaecation, ambulation, jumping and peeping became increasingly prevalent. These modifications were generally abrupt rather than progressive. Endocrine changes preceded behavioural ones which generally became evident when only two or fewer chicks remained. Conversely, the sequential capture and temporary removal of chicks from established groups in Experiment 2 elicited no significant effects in their uncaptured companions. The results are discussed in terms of social disruption, disturbance of companions by the experimenter, social motivation, predator evasion, individual recognition and fear.