Red Knot diet reconstruction revisited: context dependence revealed by experiments at Banc d'Arguin, Mauritania

Capsule Context-specific equations are needed to reconstruct diet composition and intake rate of Red Knots by the use of shell fragments retrieved from droppings.Aims To explore whether the method to reconstruct Red Knot diet described by Dekinga & Piersma [Dekinga, A. & Piersma, T. 1993. Reconstructing diet composition on the basis of faeces in a mollusc-eating wader, the Knot Calidris canutus. Bird Study 40: 144-156] accurately predicts the diet of Red Knots Calidris canutus canutus outside Northwest Europe, at Banc d'Arguin, Mauritania.Methods Feeding experiments with captive Red Knots on the bivalves Dosinia isocardia or Loripes lucinalis were carried out at Banc d'Arguin, the main wintering area of Red Knot subspecies C. c. canutus. Ingested diets were compared with the reconstructed diets derived from the general method developed by Dekinga & Piersma (1993). Droppings collected over multiple years were also analysed to evaluate the calibration method from this study.Results Of the total ingested shell mass (DMshell) in both bivalves, approximately 65% of the shell mass was retrieved in the droppings (DMdrop). Therefore, dry mass of droppings in the field (DMdrop) has to be multiplied by 1.547 to calculate the ingested dry mass (DMshell). For size estimations of ingested shells from droppings, hinges should be used for D. isocardia and hinges including tops for L. lucinalis.Conclusion The correction factor of 1.547 found here is 50% larger than the factor 0.993 for heterodont bivalves from Europe established by Dekinga & Piersma (1993). Application of the published factor would lead to serious underestimation of energy intake rates based on dropping frequencies and dropping content (by as much as 35%), although it would have small effects on the relative species composition of the diet. Having shown that such correction factors can differ among sites and prey we recommend their determination in new ecological contexts.