Waarom overwinteren sommige Drieteenstrandlopers in de Waddenzee terwijl andere tropische stranden verkiezen?

Abstract

Sanderlings Calidris alba occur worldwide along sandy beaches and have a wide range of non-breeding locations in different climate zones. Previous studies clearly indicated that ambient temperatures have profound effects on daily energy expenditures of Sanderlings. In this study we investigated the foraging ecology of Sanderlings on beaches near Esiama, Ghana and on the Wadden Sea island of Vlieland, The Netherlands. The diet was very different at the two locations. In Vlieland the birds fed predominantly on the polychaete Scolelepis squamata whereas a small shellfish Donax rugosus was taken in Esiama (Fig. 4,5). The latter prey item was very abundant in Esiama where a band of 2-14 m width containing densities of 5,700-14,300 individuals per m2 occurred. This resulted in caloric energy intake rates of 8 kJ/h foraging time in Vlieland and 130 kJ/h in Ghana. On Vlieland the birds spent on average 79% of their time foraging and in Esiama this was only 36% and most time (58 %) was spent resting (Fig. 2). We argue, however, that during the long resting breaks on roosts, Sanderlings in Ghana were in fact digesting and emptying their gut and intestines from indigestable shell remains. During future studies we plan to study annual and seasonal survival of sanderlings wintering at different latitudes in relation to disease risk and prédation probability