Possible impacts of disturbance to waterbirds: individuals, carrying capacity and populations

Disturbance to birds is defined as any human activity inducing unusual behaviour. Any deviations from undisturbed behaviour, but particularly escape responses from the disturbed site, are bound to result in extra energy expenditure. These extra costs have to be compensated for by an increase in food intake, achieved by either longer foraging bouts or by more efficient foraging. Birds disturbed while foraging suffer loss of foraging time which should also be compensated for. Because of this extra food consumption, the carrying capacity of sites for waterbirds, as expressed by the available food resources, is likely to be reduced in frequently disturbed areas. If the birds do not grow accustomed to human disturbance, repeated disturbances of areas will thus result in a lowering of their carrying capacity for waterbirds or in a decrease in body condition of the birds. Both these effects may lead to increased adult mortality, emigration, and/or decreased reproductive success and, therefore, to negative effects on population sizes. Extrapolation of effects on individual birds suggests that site or population impacts are possible. Future efforts at assessing and evaluating recreational developments in important wetlands for birds, should attempt to take these large-scale effects of disturbance into account.