Kent (1987) has shown that the chick can discriminate between its own mother and an alien hen on the basis of the hen's maternal cluck vocalization. When the cluck of the hen was examined using a sonograph it was found that there was considerable variation in the clucks produced by different hens. However the clucks remained constant in structure over time (Kent 1989). Thus the cluck is ideal for individual recognition of the mother hen by the chick. References: Kent, J.P. (1987). Behaviour, 102, 1-14; Kent, J.P. (1989). Behaviour, 108, 1-19.