Nonrandom pattern of vigilance by preening black-headed gulls

Classic models of vigilance assume instantaneous and sequential randomness in the scanning process, implying negative exponential distribution of interscan durations and no interdependence among successive interscans. We examined whether vigilance pattern by preening black-headed gulls, Chroicocephalus ridibundus, meets these assumptions. Out of 54 behavioural sequences, 50 departed from the expected negative exponential distribution, whereas the focal interscan duration was significantly affected by the interaction of the preceding scan and the interscan interval. These results reveal departures from randomness in the scanning process by gulls, which may be a consequence of the hunting strategies of their predators or due to the trade-off between the needs for feather maintenance and antipredator vigilance.