During their annual migrations, sandpipers typically gather invertebrates—such as insects, spiders, and worms—along beaches, mudflats, and shorelines in order to fuel up for their journeys. Yet, the Western Sandpiper has an additional dining technique and menu item during spring migration—this species grazes on biofilm.
Biofilm is a jelly-like substance containing a mixture of photosynthetic bacteria and other organisms, organic debris, and sediment. Since it is thick and not easily dissolved, biofilm can persist in aquatic environments. It is often a food resource for grazing invertebrates and fish but not usually for birds.
When grazing, Western Sandpipers slowly walk along dipping their heads and slightly opening up their bills to gather a bit of biofilm. Then, the head is raised and the biofilm mass is swished back and forth within the bill before being swallowed. A slow motion video clip demonstrating this grazing behavior can be found here.
The Western Sandpiper has a specialized tongue, fringed with spines and a mucous coating that aids in the gathering of biofilm from wet surfaces such as mudflats. Once a mass of biofilm is gathered, the tongue, along with bumps on the bill, help sort food from non-food. Of course, in this process, Western Sandpipers do end up ingesting sediment, which can represent up to 76 percent of their stomach contents by volume.
Despite the presence of some low-value substances, biofilm requires less effort to locate and catch than invertebrate prey, and digesting biofilm offers sugars that can be used to power migratory flights. Studies of foraging Western Sandpipers indicate that biofilm can be used to meet up to 68 percent of their daily energy requirements during migration. One study showed, perhaps not surprisingly, the distribution of foraging Western Sandpipers is more closely related to biofilm availability rather than invertebrate abundance during a migratory stopover at British Columbia’s Fraser River estuary.
Biofilm is associated with muddy marine and inland estuarine habitats with relatively calm water. Consequently, alterations of coastal conditions like sea level rise, or changes in tides or currents are potential threats to the Western Sandpiper’s habitat and food source. Other threats to Western Sandpipers include coastal development or possibly the introduction of invasive invertebrates that compete for the sandpiper’s food by also grazing biofilm.
The Western Sandpiper is a priority species for Audubon’s Saline Lakes Program. This sandpiper uses saline lakes during their migrations to and from breeding areas in Alaska and wintering areas along the south Atlantic or Gulf of Mexico coasts. While biofilm is known to occur at Great Salt Lake, no study has yet documented grazing by Western Sandpipers at inland sites, including the saline lakes of the western United States.
Audubon staff will continue working at Great Salt Lake and other saline lakes to ensure that Western Sandpipers and other shorebirds can fuel up for their next long journey.