Low dietary importance of polychaetes in opportunistic feeding Sanderlings Calidris alba on Belgian beaches

We studied abundance and feeding behaviour of wintering Sanderling Calidris alba at five beaches along the Belgian coast, and focused on the role of polychaete worms in their diet. On average 10.0 Sanderlings were present per km stretched beach, of which 87.8% were involved in foraging activities. Highest numbers of Sanderling occurred on the beach from 2 hours before to 4 hours after low tide. Detailed observations of individual birds revealed a mean pecking rate of 18.7 pecks per min. Birds exhibited several feeding modes, and we distinguished superficial pecking (mainly feeding on wreck) and probing for polychaetes. Feeding on wreck was observed frequently and throughout the tidal cycle. In contrast, probing for polychaetes was limited in time, and occurred primarily when the tide reached the upper intertidal zone, where peak densities of Scolelepis squamata were found. Based on our observations, we conservatively estimated that during each tidal cycle this benthic species may provide only 2.0-9.0% of a Sanderling's daily energy demand. Numbers of Sanderlings appeared to be lowest when capture rates on polychaetes were highest. We therefore suggest that polychaetes are of minor importance for Sanderlings foraging along Belgian beaches