SUMMARY

Although the energy costs of foraging and food processing in vertebrates may be considerable, they have rarely been quantified separately. Here we present estimates for both cost factors based on a series of trials with a shorebird, the red knot Calidris canutus, fed natural and artificial prey types under naturalistic but fully controlled indoor aviary conditions. During eight 1-day trials we successfully manipulated the extent to which the five red knots were (1) actively probing and walking (i.e. foraging) and (2) actually ingesting prey (i.e. processing food) that was (3) either hard-shelled or not (i.e. crushing). Energy expenditures, estimated by the doubly labelled water (DLW) method, calibrated for use in this particular condition, varied between 1.5 and 4 W. A hierarchical analysis of variance indicated that the crushing of hard-shelled prey entailed no extra cost. We arrived at the following breakdown of cost components under the thermoneutral conditions of the experiment: a cost of active rest/maintenance of 1.665 W, an additional cost of foraging of 0.602 W and an additional digestive processing cost of 1.082 W. These cost levels are all well within the range of expectation and are consistent with the results of a separate outdoor aviary experiment in which the thermostatic costs needed separate estimation. On the basis of the cost and performance functions of gizzards of different mass, it was shown that under the conditions of this experiment the red knots expended the bare minimum for a balanced budget, maintaining the smallest possible gizzard. Under field conditions a larger gizzard would be required.