WHY OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS CANNOT MEET THEIR DAILY ENERGY-REQUIREMENTS IN A SINGLE LOW WATER PERIOD

Captive Oystercatchers consume daily 25-40 g dry flesh or 550-850 kJ, of which they metabolize 450-700 kJ. Free-living Oystercatchers eat mo re than captive birds but, contrary to expectation, this is not due to greater activity costs but to a higher body weight. When body weights are equal, free-living and captive Oystercatchers consume the same am ount of food. The intake rate of Oystercatchers generally varies betwe en 1 and 3 mg dry flesh feeding, but if non-feeding times are included , the crude intake rate usually varies between 1 and 1.5 mg s(-1). Ext remely high intake rates, above 4 mg s(-1), are only observed in birds feeding during a short bout after a long resting period. According to Kersten & Visser (1996a) such high intake rates cannot be sustained f ur long, because a maximum of 80 g wet flesh, equivalent to 12 g dry f lesh, can be stored in the digestive tract and the processing rate doe s not exceed 4.4 mg wet flesh s(-1) or 0.66 mg ash-free dry weight (AF DW) s(-1). Due to this digestive bottleneck, the birds are forced to s pend much time on the feeding area each day. Since the exposure time o f their intertidal feeding ar eas is usually 5-6 hours, Oystercatchers cannot meet their daily energy requirements in a single low water per iod, which would often suffice if intake rate vias the limiting factor . For a given length of the feeding period, the bottleneck model predi cts the maximum crude intake intake, called CIRmax, that can be achiev ed, i.e. the highest intake rate including the non-feeding time. When the birds are able to feed for less than three hours, the achieved cru de intake rate usually remains far below this maximum, suggesting that the rare at which prey are found and eaten deter mines the intake rat e. The consumption is also usually less than would be allowed by diges tive constraint when the birds feed for twelve hours or longer, becaus e the birds at thermoneutral conditions do not need more than 36 g a d ay. When the birds spend three to twelve hours on the feeding area, th e average consumption is usually close to, or below the predicted maxi mum. However, in a few cases, the maximum was clearly exceeded. These studies do not invalidate the bottleneck model, because there is ample reason to believe that food consumption was overestimated A detailed investigation of the many sources of error indicates that food consump tion is more likely to be overestimated than underestimated in field s tudies.