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 their weight gain between two incubation spells and correcting for the amount of excreta voided before the bird returned to its nest. The model predictions agreed well with estimated food consumption based on direct observations in the field, regardless of the type of prey consumed. The model was used to calculate food consumption of free-living Oystercatchers under circumstances when this could not be measured by direct observation; (1) when feeding downshore outside their territory and (2) while feeding during night-time low water periods.

The rate of food intake outside the territory did net differ from the rate within the 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 daylight. The difference was probably induced by our activities which made the female very reluctant to incubate during the daylight hours. The average amount of food consumed differed hardly between day and night-time low water periods.

Total food consumption over a 24 hour day was 162 +/- 88 g for the male and 196 +/- 13 g for the female. Accounting for the weight loss of the male over the observation period, the estimated energy expenditure is 535 and 565 kJ day(-1) for male and female, respectively. This is equivalent to 2.2 x BMR and strongly suggests that the incubation stage is a period when energy is conserved, rather than expended.

Although the amount of food consumed per low water period varied greatly from one tidal cycle to the other, the birds appeared to balance intake with expenditure on a 24 hour basis. In the discussion we address the possible repercussions for the birds when they fail to keep this balance in the short run.