On the responses of laughing gull chicks (Larus atricilla) to the calls of adults II. Age changes and responses to different types of call

Three age-groups of laughing gull chicks were tested with playback of recordings of calls of their parents and calls of neighbouring adults. The chicks in each group showed response to the playback calls, and recognition of the voices of their parents. The chicks in the early group (1 to 3 days post-hatching) showed the least evidence of parent recognition: the chicks of the late group (12 to 28 days post-hatching) showed the lowest levels of response in general. Evidence of relationships between variation in the behaviour in the tests and variation in the test recordings was found from correlation analysis, the type of call initiating positive response, and analysis of temporal association. This evidence, and the results of tests which allowed responses to the parents' crooning and responses to the parents' ke-hah calls to be compared for chicks in the late group, indicate that whereas the filial approach behaviour of very young chicks is stimulated by crooning but suppressed by ke-hah and long-calls, this behaviour in older chicks requires the occurrence of ke-hah calls or long-calls for its elicitation. However the ke-hah calls and long-calls of adults other than the parents continue to have negative effects on the behaviour of older chicks. It is argued that this progression in discrimination of the voices of the parents and the changes in responsiveness to the different types of adult calls are consistent with functional requirements, both from the point of view of changes in a growing chick's social situation and from the point of view of the characteristics of the calls. The changes are also discussed from a developmental point of view.