Red knots scavenging on large, dying cockles:Opportunistic feeding by a sensory specialized mollusc-crushing shorebird

Despite their specializations, shorebirds can be opportunistic foragers during the non-breeding season. We describe how a highly specialized probe-feeding shorebird, the Red Knot Calidris canutus, suddenly ignored its shallow buried hard-shelled mollusc prey and opportunistically shifted to an unusual prey type: dying and dead cockles of size classes larger than 25 mm that were lying with gaping shells on the mud surface. Such prey items are normally not available because they are too large to be swallowed and too hard to be crushed in the gizzard. In the last week of August 1990, thousands of Red Knots arrived near Schiermonnikoog in the eastern part of the Dutch Wadden Sea. These birds belonged to the subspecies Calidris canutus islandica that winters at intertidal wetlands in NW Europe. During the autumn study period, the diet of the approximately 20,000 birds in our study area consisted of small shallowly buried shellfish, comprising Edible Cockle Cerastoderma edule (44.8%), Baltic Tellin Macoma balthica (36.9%) and Mudsnail Hydrobria ulvae (18.3%). In the second week of October, the diet of the complete population present shifted to dying and dead cockles lying on the surface of the tidal mudflats in a restricted area, ranging in lengths from 25 mm to 35 mm. The death of the cockles was probably due to oxygen shortage caused by the decomposition of a bloom of Noctiluca scintillans. Sampling of benthic fauna showed that the Red Knots had to deal with a relatively low availability of small profitable shellfish, which diminished in the course of the autumn as Macoma buried deeper. After one week of foraging on the opportunistic food source, with also many thousands of birds of other species such as gulls Larus spp. and Eurasian Oystercatchers Haematopus ostralegus sharing the unexpected food bonanza, almost all Red Knots left the area because the source became depleted. Foraging by knots on the dying cockles may be explained by a combination of: (1) the seasonal decline in abundance of harvestable ‘normal’ prey; and (2) the superior profitably of dying cockles over the alternative hard-shelled food items. We discuss the repercussions of the diet shift from hard-shelled mollusc to soft-bodied food with respect to gizzard atrophy.