Two types of behaviour may be distinguished during visual examination of a novel object by a chick. In one, investigation is accompanied by calls and responses such as head shaking, locomotion and wing flapping; in the other, investigation is silent and response is inhibited, so that at the most extreme even head movements are absent. The types occur in phases. After the introduction of a small novel object, the first phase predominates in testosterone-treated chicks (T’s), the second in controls (C’s). In T’s, investigation with calling tends to occur in bouts. Head shaking occurs at transition points between one kind of behaviour and another. Such transitions are probably commoner during investigation than in C’s, but this only partly explains the greater frequency of head shaking in T’s. The complex of behaviour occurring during investigation with calling may be most nearly compared with low intensity mobbing. Testosterone seems to facilitate it specifically; there is, for example, no such effect on escape and associated responses. Other factors affecting the balance between these two phases of investigation are reviewed.
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