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

Quantitative Relations between Prey Quality, Gizzard Mass, and Local Survival Rate

<div><p>(A) Gizzard mass required to maintain energy balance declines as a function of prey quality (solid line). Gizzards of birds seen again after catching fit this relationship (gray dots; mean ± SE), whereas gizzards of birds not seen again are significantly smaller (open dots; note that both groups almost entirely overlap in the poorest-quality year).</p> <p>(B) Observed gizzard masses upon arrival are distributed according to the left-most normal distribution. If there were no room for flexibly adjusting gizzard mass (0 g), only 47% of the arriving knots would be able to avoid starvation (shaded area) at an example prey quality of 0.15 g flesh per g shell (dashed line). If there were room for flexibility (+1 and +2 g in this example), a much larger proportion would be able to survive (respectively 70% and 88%).</p> <p>(C) Observed local survival rate (y⁻¹; ± SE) increased as a function of prey quality and best matched with predicted survival in a +1-g flexibility scenario.</p></div

Changes in Suitability, Prey Quality, and Gizzard Mass

<div><p>(A) The percentage of blocks that yielded insufficient intake rates (<4.8 W) increased over time due to an increase in unsuitable blocks being dredged in previous years (dark gray bars; as opposed to light gray bars indicating unsuitable blocks that were never dredged).</p> <p>(B) Quality of prey included in the diet has declined over time (box-and-whisker plot, line gives GLM ± 95% confidence intervals).</p> <p>(C) In response, gizzard mass has increased over time (boxes and lines as in (B)).</p></div