FOOD-INTAKE OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS BY DAY AND BY NIGHT MEASURED WITH AN ELECTRONIC NEST BALANCE

We developed a model to calculate food intake by Oystercatchers from t heir weight gain between two incubation spells and correcting for the amount of excreta voided before the bird returned to its nest. The mod el predictions agreed well with estimated food consumption based on di rect observations in the field, regardless of the type of prey consume d. The model was used to calculate food consumption of free-living Oys tercatchers under circumstances when this could not be measured by dir ect observation; (1) when feeding downshore outside their territory an d (2) while feeding during night-time low water periods. The rate of f ood intake outside the territory did net differ from the rate within t he territory and averaged 0.9 g min(-1) fresh weight. Food intake rate in darkness did not differ from that during daylight (p = 0.96). The total amount of food consumed per low water period et night was higher in the male and lower in the female as compared to food consumed in d aylight. The difference was probably induced by our activities which m ade the female very reluctant to incubate during the daylight hours. T he average amount of food consumed differed hardly between day and nig ht-time low water periods. Total food consumption over a 24 hour day w as 162 +/- 88 g for the male and 196 +/- 13 g for the female. Accounti ng for the weight loss of the male over the observation period, the es timated energy expenditure is 535 and 565 kJ day(-1) for male and fema le, respectively. This is equivalent to 2.2 x BMR and strongly suggest s that the incubation stage is a period when energy is conserved, rath er than expended. Although the amount of food consumed per low water p eriod varied greatly from one tidal cycle to the other, the birds appe ared to balance intake with expenditure on a 24 hour basis. In the dis cussion we address the possible repercussions for the birds when they fail to keep this balance in the short run.