QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online August 25, 2003

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JEB Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by van Gils, J. A. Articles by Dietz, M. W. Search for Related Content PubMed PubMed Citation Articles by van Gils, J. A. Articles by Dietz, M. W.

Cost-benefit analysis of mollusc-eating in a shorebird II. Optimizing gizzard size in the face of seasonal demands

Jan A. van Gils^1,2,*, Theunis Piersma^1,2, Anne Dekinga¹ and Maurine W. Dietz²

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 (CEES), University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands

View larger version (13K): [in a new window]   Fig. 1. Frequency distribution of gizzard mass (g) observed on free-living red knots throughout the years 1984-2002 in the Wadden Sea (N=920, of which 73 were obtained through dissection of carcasses and 847 through ultrasonography on live birds).

View larger version (16K): [in a new window]   Fig. 2. Gizzard mass estimated ultrasonographically in the three experiments as a function of the hardness of the staple food on offer. Values are means ± S.E.M. In all three experiments gizzard size manipulations were successful: the effect of the hardness of the diet on gizzard mass is significant (P<0.01).

View larger version (21K): [in a new window]   Fig. 3. Intake rates of the bivalve prey types in experiment 1 as a function of a prey type's shell mass (values are means ± S.E.M.). Prey types are three different size classes of Macoma (triangles) and Cerastoderma (circles). Symbols are open for the small-gizzard birds and filled for the large-gizzard birds. Solid lines are the linear regression lines with fixed shell mass processing rates for each group of birds (0.24 and 2.58 mg s-1, respectively, for the small- and large-gizzard birds; model 4 in Table 2). Broken lines give observed handling rates (1/H) for Macoma and Cerastoderma. Squares denote measurements on red knots in the field by (1) González et al. (1996) and (2) Zwarts and Blomert (1992). Grey diagonal `Kirkwood-Kvist' bar indicates the constraint on metabolizable energy intake rate according to Kirkwood (1983) and Kvist and Lindström (in press; average of the two predictions is used). The bars on top of the graph denote the relative frequency distribution of shell masses of ingestible bivalve prey in the western Dutch Wadden Sea (N=82 964).

View larger version (15K): [in a new window]   Fig. 4. Intake rate in experiment 2 as a function of gizzard size class (values are means ± S.E.M.). Open circles indicate a soft diet of unshelled mussels was offered; closed circles, a hard diet of with-shell mussels was offered. The broken line gives observed handling rate (1/H). The broken diagonal line gives the intake rate for with-shell mussels predicted from shell mass, gizzard size and regression coefficients obtained in experiment 1. The grey bar gives the postulated maximum metabolizable energy intake rate [average of Kirkwood (1983) and Kvist and Lindström (in press)].

View larger version (33K): [in a new window]   Fig. 5. Daily intake in experiment 3 increases as a function of the daily available foraging time Tn. Values are means ± S.E.M.; intake is expressed both as number of prey (left axis) and in metabolizable energy intake (right axis). The rate of increase (i.e. the intake rate) is similar across the three treatments (2, 6 and 16 h; P>0.95), and is correctly predicted by shell mass per prey and the flocks' average gizzard mass (G=8.13 g; broken line based on the parameters of experiment 1; P>0.85), and is much lower than the rate of prey-handling (1/H, broken line; P<0.001). These observed intake rates were close to the postulated upper limit (grey bar; Kirkwood, 1983; Kvist and Lindström, in press). The thick solid line gives daily expenditure for G=8.13 g. The experimental birds would just balance their daily energy budget when feeding for 12 h (arrow), which is exactly the time that is naturally available in their intertidal habitats. If the birds had had smaller gizzards (thin solid lines indicating gizzard mass G in g), they would have needed more time for this (even though their daily requirements would go down somewhat - this is not plotted here, but see Piersma et al., 2003).

View larger version (28K): [in a new window]   Fig. 6. (A)Diet composition (stacked bars scaled onto left axis; expressed as a percentage of total energy consumption) and the monthly-specific amount of flesh mass per prey type (Zwarts, 1991; not plotted here) determine the quality of the average ingested prey (filled circles scaled onto right axis, expressed as the amount of metabolizable energy per g shell mass, DMshell). The data plotted here are for red knots living in the Wadden Sea (1988-2000). (B). For satisficing knots, prey quality (denoted by diagonal lines of equal prey quality) together with the daily amount of energy required to balance the energy budget in the Wadden Sea (horizontal axis) predict for each month the gizzard size (right axis) that is required to process the daily amount of shell material (left axis). Alternatively, for net rate-maximizing knots, prey quality together with the maximum amount of energy that can be assimilated on a daily basis (given by the vertical Kirkwood-Kvist bar) predict for each month the required gizzard mass. (C). Predicted gizzard masses for satisficing and net rate-maximizing red knots (lines) overlaid with data on gizzard masses of free-roaming red knots in the Wadden Sea in 1984-2002 (values are means ± S.D.; N=920, of which 73 were obtained through dissection of carcasses and 847 through ultrasonography on live birds). Net rate-maximizing gizzards are found in spring (February-May), while satisficing gizzards are found throughout the remainder of the year (July-January).