In the spring of Georgia Strait, Canada, the mudflats south of Roberts Bay become an open-air dining room for West Coast Sandpipers at low tide each day. The western sandpiper is a small wading bird (generally refers to long-legged birds that live on the shores of water bodies), and they have strong migratory abilities. Every year, hundreds of thousands of western sandpipers migrate from Panama to Alaska to breed, and Roberts Bay is their last stop before reaching their breeding grounds in Alaska. In order to maintain enough energy to complete the trip north, the sandpiper needs to rest and eat in Roberts Bay. Strangely, the birds spent most of their time along the mudflats sucking the sticky mud on the ground, in addition to running back and forth, much like marathon runners sucking on energy gels to replenish their energy.
Mud is also delicious
Scientists once thought that the mud-sucking sandpiper was trying to find aquatic insects, crustaceans, mollusks and other invertebrates. But later, they found that these west coast sandpipers were actually sucking a snot-like film on the surface of the mud—a biofilm composed of photosynthetic bacteria, algae and other microorganisms and their secretions.
A West Coast Sandpiper is feeding on a biofilm
The tongue of the western sandpiper seen under the microscope
A West Coast Sandpiper resting in Roberts Bay
So how do these biofilms form? Why do west coast sandpipers suck them? It turns out that some “witty” photosynthetic bacteria, algae and other microorganisms secrete mucus to attach themselves to the surface of the mud, so that they can temporarily settle down and not be swept away by the sea water. Don’t look at these biofilms like sticky snot, they are a nutrient-rich delicacy for the West Coast Sandpiper. Although the sandpiper will inevitably eat some sediment when sucking the biofilm on the surface of the mud, these biofilms are more easily located and preyed by the sandpiper than looking for invertebrates, thus saving a lot of physical effort.
So how do western sandpipers scrape these biofilms? Scientists have observed under a microscope that the outer edge of the sandpiper’s tongue is covered with many tiny “burrs”. These “burrs” make the sandpiper’s tongue look like an old shag carpet or an overused toothbrush head. It is with these “burrs” on the outer edge of their tongues that the western sandpiper scrapes the biofilm. Scientists surveyed six intertidal zones (the coast between the average highest and lowest tides) in Japan and Canada, and after studying 30 different species of shorebirds, they found that not just west shore sandpipers, but almost all small waterbirds Both use their beaks and furry tongues to suck up biofilms, and these smaller birds have more developed “burrs” on their tongues. Smaller birds have difficulty digesting bulky or hard food because their digestive organs are too small, and their beaks are usually short, making them unable to hunt in deep sediments. Scientists theorize that these smaller birds evolved their tongue morphology and foraging behavior to prey on smaller, softer foods, such as biofilms.
Not all mudflats are patronized
The biofilm on the mud surface is not only rich in nutrients, but also provides energy for the west coast sandpiper to continue its northward migration. This is because, under the right conditions, these biofilms photosynthesize and produce sugars and fatty acids. Studies have shown that the energy provided by the biofilm can meet up to 68% of the energy requirements of the western sandpiper during migration.
Part of the “menu” of the West Coast Sandpiper
Strangely, in the mudflat dozens of square kilometers south of Roberts Bay, the western sandpiper is mainly concentrated in the supratidal area only 300 meters from the coast. By classifying the stomach contents of the sandpiper and documenting its foraging behavior, scientists found that this area is the place where the biofilm is thickest, most nutritious and exposed for the longest time. Because the area is at the mouth of the Fraser River, where freshwater mixes with seawater, it creates just the right level of salinity for the halophyte algae, prompting them to produce large amounts of fatty acids. Polyunsaturated fatty acids play an important role in the health of organisms, so the western sandpiper needs to supplement this substance through food. As a result, the Westshore Sandpiper became choosy about the “abundant” biofilms on the mudflats. For example, biofilms in some bays contain higher proportions of polyunsaturated fatty acids, and more of the biofilms will be absorbed by the sandpiper that stays there; while biofilms in some bays contain fatty acids with lower nutritional value. When the proportion is larger, the Western Sandpiper is less likely to see these snot-like substances on the mudflat, but spends more time preying on polychaete worms (they contain polyunsaturated fatty acids).
The West Coast Sandpiper is one of the most abundant shorebird birds in the world. Scientists speculate that the staggering number of sandpipers may be related to their feeding on biofilms, a nutrient-rich food resource. Without these slimy biofilms, most sandpipers would not be able to make long migrations. So it seems that the biofilms in these slurries make a huge contribution.