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, DM_shell). 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).

Return to article