Cost–benefit analysis of mollusc eating in a shorebird : I. Foraging and processing costs estimated by the doubly labelled water method

Although the energy costs of foraging and foodprocessing in vertebrates may be considerable, they haverarely been quantified separately. Here we presentestimates for both cost factors based on a series of trialswith a shorebird, the red knot Calidris canutus, fednatural and artificial prey types under naturalistic butfully controlled indoor aviary conditions. During eight 1-day trials we successfully manipulated the extent to whichthe 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 thedoubly labelled water (DLW) method, calibrated for usein this particular condition, varied between 1.5 and 4·W. Ahierarchical analysis of variance indicated that thecrushing of hard-shelled prey entailed no extra cost. Wearrived at the following breakdown of cost componentsunder the thermoneutral conditions of the experiment: acost of active rest/maintenance of 1.665·W, an additionalcost of foraging of 0.602·W and an additional digestiveprocessing cost of 1.082·W. These cost levels are all wellwithin the range of expectation and are consistent with theresults of a separate outdoor aviary experiment in whichthe thermostatic costs needed separate estimation. On thebasis of the cost and performance functions of gizzards ofdifferent mass, it was shown that under the conditions ofthis experiment the red knots expended the bare minimumfor a balanced budget, maintaining the smallest possiblegizzard. Under field conditions a larger gizzard would berequired.