Vigilance Decreases with Time at Loafing Sites in Gulls (Larus spp.)

Understanding how animals partition effort between vigilance for predators and other conflicting activities has been a mainstay of animal behaviour research. Classical theories implicitly assume that animals alternate between high and low vigilance states over short timescales, but that average effort invested in vigilance is constant over an extended bout of such alternations. However, one recent model suggests that vigilance should be adjusted dynamically to short-term changes in the perception of predation risk and would tend to decrease with time. Indeed, as time passes by without disturbances, perception of the need for vigilance should decrease and prey animals may allocate more time to competing activities. Here, we examined how the proportion of sleeping gulls (Larus spp.) in loafing groups changed over time. Sleeping gulls can only maintain low levels of vigilance against external threats (compared to alert individuals), and we predicted that the proportion of sleeping gulls at loafing sites should increase over time when no disturbances occur. Statistically significant changes in the proportion of sleeping gulls as a function of time occurred in the majority of sequences and an increase was observed significantly more often than predicted by chance alone. This temporal pattern cannot be caused by reduction in hunger levels because gulls are not feeding at loafing sites. The results indicate that vigilance can be adjusted dynamically in response to short-term temporal changes in the perception of predation risk.