Compromise between seabird enjoyment and disturbance:: the role of observed and observers

Natural areas are increasingly visited by people, and urban human visitors expect to watch wildlife as close as possible, but this may have associated disturbance costs. Here, effects of number of visitors and bird density on flight initiation distance (FID) as a proxy of disturbance vulnerability were evaluated in the large ground-nesting yellow-legged gull, Larus michahellis. Mean FID decreased with increasing number of visitors and with increasing gull densities, suggesting that (1) ground-nesting gulls habituate to massive human presence, while retaining their antipredatory mechanisms, and (2) dense groups Of gulls were more reluctant to fly away. This density effect may be due to the increased risk Of Clutch predation by conspecifics at high densities and, if so, FID is a reliable metric of disturbance vulnerability in groundnesting gulls. In conclusion, set-back distances are specific to local populations and it is unnecessary to ban or restrict human visits to ground-nesting gull colonies; redistributing visits, taking into account both the number of visitors and gull density, is preferable.