Abstract
Sequences of vigilance behaviour were obtained for preening crested terns, Sterna bergii. Paired comparisons were made between the vigilance of individuals before and after flock size changes. As predicted, individuals responded to flock size increases by increasing their inter-scan intervals (reducing vigilance), and to flock size decreases by decreasing inter-scan intervals. Scan durations did not vary with flock size. However, arrivals and departures of birds were immediately followed by long scans. The study represents a more powerful test of flock size effects than has previously been presented in correlational studies.
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