Primary data used in Mathot, Dekinga and Piersma, Functional Ecology

The attached file includes three worksheets: 1) gizzard mass data, 2) ad libitum foraging trial data, and 3) diet choice trials. Complete descriptions of all column headings are provided on the first tab of the excel file. For details of the methodology, please refer to the original publication in Functional Ecology

Regression of DM_shell intake on non-toxic prey against gizzard mass.

<p>Data from this study on <i>Dosinia</i> was combined with data from van Gils <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.ref014" target="_blank">14</a>] on other non-toxic prey species. Adding the current data to the regression derived by van Gils <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.ref014" target="_blank">14</a>] slightly changes the regression line (though not significantly; from dashed to solid line), but greatly reduces standard error (from light to dark grey area). Parameter estimates are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.s006" target="_blank">S1 Table</a> (model 3.1). Regressions are linear regressions on log-transformed data. Note that van Gils <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.ref014" target="_blank">14</a>] averaged gizzard mass measurements per bird, whereas we estimated gizzard mass in each trial by interpolating measurements.</p

Appendix A. On the calculation of ∆x.

On the calculation of ∆x

Second-order Akaike’s information criterion (AIC_c) comparison of statistical models.

<p>Model selection based on AIC_c, with a penalty of 2 per added parameter [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.ref029" target="_blank">29</a>]. Models are ordered by adequacy, starting with the minimum adequate model. Model 1.2 is competitive with model 1.1. Model 2.1 and 3.1 do not have competitors. All models are linear mixed models with a Gaussian error structure, and contain bird ID as a random effect. Models 2.1 to 2.5 contain a variance structure based on prey species.</p><p>^a In model 1.1 to 1.5, factor “diet” refers to the diet outside the experimental trials, being either soft or hard-shelled. Factor “group” refers to the order of these diet treatments (group 1 or group 2). In models 2.1 to 2.5, factor “gizzard” refers to gizzard size during the trial, which was either small or large; “species” refers to the prey species being offered, which was either <i>Dosinia</i> or <i>Loripes</i>. In models 3.1 to 3.5 log(gizzard) is a continuous variable that refers to the logarithm of estimated gizzard mass during the trial; species refers to prey species, which was either <i>Dosinia isocardia</i>, <i>Cerastoderma edule</i> or <i>Macoma balthica</i>. The symbol × means that the main terms as well as their interaction are fixed effects in the model. Models 1.4, 2.4 and 3.5 contain only an intercept, no fixed effects.</p><p>^b The number of parameters in the model.</p><p>^c Log likelihood.</p><p>^d Dry ballast mass.</p><p>Second-order Akaike’s information criterion (AIC_c) comparison of statistical models.</p

Experimental studies on gizzard size and diet in red knots.

<p>Experimental studies on gizzard size and diet in red knots.</p

Dry shell mass (DM_shell) intake rate on a <i>Dosinia</i> diet (A) and on a <i>Loripes</i> diet (B).

<p>Lines connect all trials of the same bird when it was in the small gizzard group (open dots) and in the large gizzard group (solid dots). Intake of <i>Dosinia</i> was higher for birds with large gizzards, whereas intake of <i>Loripes</i> was not affected by gizzard size (model 2.1 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.s006" target="_blank">S1 Table</a>). <i>Loripes</i> intake rate was generally lower than <i>Dosinia</i> intake rate. These results confirm that intake of <i>Dosinia</i> is limited by a digestive constraint, whereas intake of <i>Loripes</i> is limited more stringently, presumably by its toxic load, and independent of gizzard mass.</p

Appendix B. A figure showing availability of feeding sites as a function of tidal time and cycle type.

A figure showing availability of feeding sites as a function of tidal time and cycle type

Mean gizzard mass of birds directly after catch, during the first and second series of trials.

<p>Directly after catch, the 6 red knots were randomly divided into two groups, group 1 (solid dots and line) and group 2 (open dots and dashed line). Both groups received different diets outside the experimental trials (soft or hard-shelled prey) to manipulate gizzard size. Initial differences in gizzard mass between groups were not significant (F_1,4 = 3.9,p = 0.12). After catch, all birds decreased gizzard mass, but group 1 had larger gizzards than group 2 during the first series of trials, and smaller gizzards during the second series (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136144#pone.0136144.t002" target="_blank">Table 2</a>, models 1.1 to 1.5), showing that the manipulation of gizzard size was successful. Each group consisted of three birds. However, data collected on one bird from group 2 after it became sick during series 2 was omitted from the graphs and the analysis. Error bars show standard error.</p

Conceptualization of Why a Simultaneous Reduction in Density and Quality of the Prey is Detrimental

<p>Holling's Type II functional response describes intake rate (be it flesh or energy) as a function of the density of either poor-quality (black lines) or good-quality (gray lines) prey. Digestive constraint limits shell-mass processing rate and is given for two gizzard sizes for each prey quality (horizontal cut-offs in functional response; digestively unconstrained intake rates continue as dashed lines). By knowing the threshold intake rate needed to avoid starvation (border between gray and white background), one can predict a bird's starvation chances on the basis of gizzard size and prey quality and density. (1) A small gizzard is sufficient to stay alive when prey is of good quality and occurs in high densities. Going from (1) to (2), prey density is reduced, which does not affect survival as intake rate remains above the critical threshold. Going from (1) to (3), prey quality (flesh-to-shell ratio) is reduced. To maintain a sufficient intake rate, the knot needs to increase its shell-mass processing rate, which requires a gizzard enlargement. Going from (1) to (4), the combined reduction in density and quality makes a gizzard enlargement no longer sufficient (as intake rate is now constrained by prey density), and the bird is bound to starve.</p

Study Area and Effects of Dredging

<div><p>(A) Map of the study area with 2,846 sampling stations (dots) categorized into 272 square kilometer blocks (squares containing 16 stations at most). A dot is filled when a station has been dredged at least once in 1998–2002 and is open when the station was never dredged during that period.</p> <p>(B) Densities of available cockles remained stable in dredged blocks, but they increased (+3% y⁻¹) in undredged blocks (open dots ± SE bars). Quality of available cockles declined in dredged areas (−11% y⁻¹), whereas it remained stable in undredged areas (filled dots ± SE bars).</p></div