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First published online August 25, 2003

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

Theunis Piersma^1,2, Anne Dekinga¹, Jan A. van Gils^1,2,*, Bart Achterkamp² and G. Henk Visser^2,3

¹ Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands
² Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands
³ Centre for Isotope Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

View larger version (14K): [in a new window]   Fig. 1. Methodological graph showing the degree of success of the three different treatment categories: (A) fraction of time spent foraging, (B) measurement of intake rate (ash-free dry mass MAFD per second), as an indication of the extent of energy intake, and (C) measurement of intake rate (shell dry mass DMshell per second) to indicate shell crushing. These box-and-whisker plots give median (horizontal line within box), interquartile range (box), range (bars), and outliers (small filled circles) among all relevant trials. Larger symbols indicate mean values per individual per treatment category.

View larger version (11K): [in a new window]   Fig. 2. Rates of energy expenditure (W) measured with the doubly labelled water method as a function of whether the birds were foraging (horizontal axis), subdivided into trials when they were not crushing or processing digesta (dark grey boxes), processing but not crushing (open boxes), or processing and crushing (light grey boxes). Large filled circles are least-square means and small filled circles are outliers (see Fig. 1 for a further description of box-and-whisker plots).

View larger version (23K): [in a new window]   Fig. 3. A comparison of the estimated metabolic rates found in this indoor study (filled circles) with a doubly labelled water-estimate obtained in a similar study on an outdoor intertidal flat (Poot and Piersma, 1994; filled triangle). In this outdoor study it was calculated that the thermostatic costs amounted on average to 0.88 W. Thermostatic costs were absent in our indoor study. We therefore subtracted 0.88 W from the outdoor metabolic rate (open triangle) to make the metabolic rates from both studies comparable (note that this assumes the additive energy budget model is operative - see main text). This `thermoneutral' outdoor metabolic rate is correctly predicted by combining the three indoor activity-specific cost estimates with the outdoor time budget (7.2 h of available foraging time per day, of which 71% was actually spent foraging). Values are means ± S.E.M. HIF, heat increment of feeding.

View larger version (30K): [in a new window]   Fig. 4. Energy budget for knots feeding on poor-quality cockles as a function of gizzard size. For the experimental birds, gizzard mass = 8.13±0.98 g (mean ± S.E.M.), we plotted the cumulative cost levels for the three components that we measured (rest, foraging and heat increment of feeding, HIF; filled circles; figures as in Table 2B).Values are means ± S.E.M. Theoretically, each of these cost levels would increase with gizzard mass (with concomitant increases in intestine mass, Table 3), since larger gizzards require larger maintenance costs (Piersma et al., 1996), larger gizzards increase the cost of walking (Bruinzeel et al., 1999), and larger gizzards quadratically increase HIF (since intake increases quadratically with gizzard size; see van Gils et al., 2003a). Two digestive constraints are presented: (1) the gizzard-size independent rate at which flesh can be digested (horizontal grey bar; based on Kirkwood, 1983 and Kvist and Lindström, in press), and (2) the rate at which shell material is processed, a quadratic function of gizzard size (solid curve; van Gils et al., 2003a). Intake rates measured in the two most `natural' trials (means of trials IF-C1 and IF-C2; open square) appear to be set by this `gizzard-size constraint'. The hatched area below the two digestive constraints but above the cumulative cost levels gives the scope for a positive energy budget and is maximal at the arrow.