HUMAN DISTURBANCE AFFECTS PARENTAL CARE OF MARSH HARRIERS AND NUTRITIONAL-STATUS OF NESTLINGS

Most studies of the effects of human disturbance on reproduction in bi rds of prey have not quantified the subtle and potentially important a spects of such interactions. Thus, we studied the effects of human dis turbance on parental care by marsh harriers (Circus aeruginosus) in sp ring 1991 at Dos Reinos Lake, Ebro Valley, Spain. Specifically, we ass essed changes in reproductive activities and nutritional condition of nestlings due to low-level human disturbance during incubation and nes tling phases. The number of food items delivered and the time spent by males and females in the nesting area and on the nest decreased durin g disturbed periods, especially during incubation (P < 0.001), whereas behaviors related to stress (alarm calls, chases against other intrud ing birds, and percentage flying time) increased (P < 0.001). Although annual productivity of the disturbed pairs was not affected, nestling s of disturbed birds exhibited levels of blood urea that were higher ( P < 0.001) than those of undisturbed pairs. Thus, minor human disturba nces may cause long-term effects on lifetime reproductive success of b irds by increasing energy and time expenditure in non-reproductive act ivities and by reducing condition of nestlings.