04/22/2026
Happy Earth Day! Mud Flats have once again been a hot topic as we get closer to the removal of the 5th Ave Dam. Revisit this article with us and take a moment to appreciate our Mud Flats as the bedrock of a vibrant ecosystem they are.
Happy Earth Day!! Celebrate with a Little MUD!! DERT Loves Mud!!
Mudflat Meals...a story By Peter Yager
Have you ever been out on the Estuary Boardwalk Trail at Billy Frank Jr. Nisqually National Wildlife Refuge or the Sandpiper Trail at Grays Harbor NWR at low tide? Past the salt marshes you’ll see acres and acres of desolate-looking mudflats covered in nothing but slime and smelling like rotten eggs. The barren-looking mudflats of the Nisqually and Chehalis River deltas are the furthest thing from desolate. The sediment deposited in estuaries is incredibly nutrient-rich and teeming with animal life. One square yard of mudflats can contain 100 clams, 2,000 worms and 30,000 amphipods!
The richness of mudflat life plays a critical role in the annual shorebird migration. Shorebirds make some of the most incredible migrations of any species. The red knot travels over 10,000 miles from its wintering grounds in Tierra del Fuego, Argentina, to its nesting grounds in the high Arctic. In order to make these incredible journeys, shorebirds need to have abundant food available in their wintering grounds and stopover points. Some shorebirds can gain half their body weight or more in body fat during the winter.
The timing of the spring shorebird migration is critical. The entire breeding population needs to arrive on the nesting grounds within a few days of each other in order to find mates and raise chicks during the short Arctic summer. This means that a few special mudflats will harbor over a million shorebirds at a time during the spring migration. On the Pacific Flyway, Grays Harbor and the Copper River Delta in Alaska are the critical stopover points. This peak of migration occurs around the beginning of May at Grays Harbor and is commemorated by the Grays Harbor Shorebird and Nature Festival.
After their chicks fledge, the shorebirds can trickle south with less urgency, so the numbers of migrating birds are less dramatic in late summer and fall. Both Grays Harbor and the Nisqually Delta are also home to several species of wintering shorebirds.
So what exactly are all those birds doing out there in the mud? Every shorebird is superbly adapted to feeding on mudflats and adapted to a certain type of prey. Their exact feeding pattern can be guessed by looking at their legs, eyes, and most importantly, their bills.
Plovers, like the Black-bellied Plover and Semi-palmated Plover, have shorter bills and large eyes. These birds are visual hunters who pick up larger prey on mud or beach surfaces like marine worms, insect larvae, amphipods, and shrimp.
The Greater Yellowlegs (and the closely related Lesser Yellowlegs) have some of the longest legs of our common shorebirds and slightly upturned bills. They use their long legs to wade in a few inches of water and use their upturned bill to scoop floating prey out of the water.
Ruddy Turnstones have short slender bills. As their name implies, they flip over small rocks on beaches and catch invertebrates hiding underneath.
Short-billed and Long-billed Dowitchers wade in shallow water stabbing the mud with a sewing machine-like motion. Their bills, like those of many shorebirds, are semi-flexible and sensitive enough to detect small crustaceans and clams hiding in the mud.
Red Knots and Marbled Godwits have similar long bills as the Dowitchers but prefer feeding on slightly higher ground instead of in the water probing deeply for immature clams, crustaceans, and worms. Marbled Godwits, unlike most Washington shorebirds, nest on the plains of Montana and Alberta instead of in Alaska and the Canadian Arctic. Red Knots are most common on the Atlantic Flyway, where they time their migration with the moon so they can fatten up on the eggs of breeding horseshoe crabs in Delaware Bay.
The Whimbrel has one of the most dramatic bills of any shorebird. It uses its long curved beak to probe deep in the mudflats for burrowing worms and crustaceans. The closely related Long-Billed Curlew, which can occasionally be seen wintering in Washington, has an even longer bill!
The vast majority of the birds that are seen migrating through Grays Harbor or wintering in Western Washington are small sandpiper species like Western Sandpipers, Dunlin, and Least Sandpipers. Their bills are not as long as the mud-probing birds or as stout as the birds that catch larger prey. These tiny birds actually get a significant amount of their nutrition from scum!
The scum that you see on tidal mudflats is called biofilm. It’s a mat of diatoms (a type of plankton), organic and inorganic detritus, and bacteria held together by mucosal secretions to form an evocatively-named mucilaginous matrix. The nutritious sediment feeds the biofilm, and the biofilm feeds the small creatures of the mudflats. The thousands of clams, worms, insects, and crustaceans that feed the larger shorebirds feed on biofilm and so do sandpipers. A study in British Columbia estimated that up to 70% of the diets of Western Sandpipers consisted of biofilm.
Sandpipers have flexible bills and spiny tongues, which allows them to scoop up biofilm as easily as they pick up tiny amphipods and other invertebrate life. Sandpipers travel in large flocks for defensive purposes, running in large groups following tides. Western sandpipers run after outgoing tide picking up invertebrates and biofilms as they are beached and least sandpipers forage on the upper ends of mudflats against the salt marsh vegetation. Dunlin run along mudflats with a rapid pecking motion and are easily spotted when they take to the air, flashing their distinctive white wing patches in synchronized aerial maneuvers to evade predators like Peregrine Falcons.
So next time you look at a tidal mudflat, whether or not you see shorebirds, take time to appreciate the diversity and richness of these unique habitats. Mudflat slime isn’t gross and smelly; it’s the base of a food pyramid that a huge number of land and sea species depend on.
