Sediment in Stomach Contents of Western Sandpipers and Dunlin Provide Evidence of Biofilm Feeding

—Western Sandpipers and Dunlin are capable of grazing biofilm. As there has been no assessment of this dietary constituent in stomach contents, the stomachs of 89 Western Sandpipers and 56 Dunlin collected during breeding migration through the Fraser River delta, British Columbia, Canada, were examined. Invertebrates, traditionally regarded as the principal prey, comprised a minor fraction of mean stomach volumes (Dunlin: <25%; Western Sandpiper <10%). Three phyla accounted for most of these invertebrates: molluscs, annelids and arthropods. In comparison, sediment (a mixture of sediment particles, broken and unbroken diatoms plus organic detritus) comprised the major component of stomach volumes (Dunlin: >40%; Western Sandpipers: >75%). Although the mean volume of sediment was significantly greater in Western Sandpipers than Dunlin, there was no effect of sex for either species. Stomach sediment volume appears a convenient index of biofilm intake and sediment loads indicative of biofilm grazing have been reported in stomach contents from other shorebird species. Re-examination of shorebird diets appears necessary given that conditions promoting biofilm are not necessarily conducive for invertebrate prey. Received 6 October 2009, accepted 25 May 2010.