The Rate of Food Processing in the Oystercatcher: Food Intake and Energy Expenditure Constrained by a Digestive Bottleneck

Whether food intake is determined by the maximum rate at which animals can collect food, or by the rate at which this food can be processed, will strongly affect the organization of their behaviour. We investigated whether the digestive system imposes a constraint on (I) instantaneous rate of food acquisition, (2) total daily food consumption and (3) sustainable maximum energy expenditure in the Oystercatcher, Haematopus ostralegus. We measured the rate of food processing in captive birds from the cumulative loss of body mass after the consumption of various amounts of mussels, Mytilus edulis. 2, Mass loss associated with the digestion of food can be conveniently described by a three-parameter model. These parameters are: (1) latency time, the time interval that elapses between the ingestion of the first food item and the production of the first elapses (28.3 min), (2) evacuation rare, the rate with which digested fod is defecated (0.233 g min(-1)) and (3) absorption coefficient, the proportion of the food (fresh mass) which is not defecated (11.3%). The value of all three parameters was independent of the amount of food consumed, 3, Since the birds absorbed 11.3% of the food, the rate of food processing is slightly higher than the evacuation rate and amounts to 0.263 g min(-1). 4. Given the energy content of the food (3.58 kJ g(-1) fresh mass) and an assimilation efficiency of 85.4%, the instantaneous rate of energy assimilation is 13.4 W. 5. An 80-g food storage capacity in the oesophagus enables the bird to continue digestion during most of the high-water roosting period. Consequently, the maximum amount of energy that can be assimilated during a 24-h day with two low-water feeding periods amounts to 1067 ELT, which equals 4.25 times the basal metabolic rare. This figure probably reflects the maximum sustained working level. 6. Free-living Oystercatchers collect food much faster than they are able to process it. A digestive bottleneck forces them to interrupt their feeding at regular intervals, even when they attempt to collect the maximum amount of food that can be dealt with by the digestive tract. The main implication of such a digestive bottleneck is that a considerable amount of time becomes available for activities other than feeding. In practice, mast of this spare time is spent inactive and we will point out how some evolutionary oddities, such as laziness and individual differences in prey preferences, may have evolved as by-products of a digestive bottleneck.