FEEDING-BEHAVIOR OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS DURING A PERIOD OF TIDAL MANIPULATIONS

Due to the construction of a storm surge barrier in the mouth of the O osterschelde, the tidal amplitude in the estuary can be manipulated. I n this paper I describe the effects of a closure during the autumn of 1986 on the numbers, distribution and foraging behaviour of Oystercatc hers. During the closure, which lasted for six consecutive tides, the water level remained high and only the upper part of the intertidal ar ea was exposed. Oystercatchers spent most of their time feeding in thi s zone, where they normally spent only a very short time during their movement down to the preferred feeding areas below mid-tidal level. Wh en feeding conditions further deteriorated during a severe gale, birds stopped feeding altogether. When the tidal regime was restored, birds distributed themselves over the feeding areas as before. It was calcu lated from measurements of food intake that, during the closure of the barrier, a bird, on average, obtained only two thirds of the food tha t it would have taken under normal conditions. If Oystercatchers are a ble to increase their intake rates above normally observed levels, the y would have been expected to do so as soon as they were able to feed again on their preferred feeding areas. However no change in intake ra tes took place, indicating that Oystercatchers generally feed at their maximum rates. This contrasts with some experimental results. Total n umbers of birds in the study area during this period of closure were n ot affected, although the consumption deficit could have influenced th eir condition.