This area is rich in invertebrates, including worms, clams and crustaceans (crabs).  One square yard of mudflats can contain 100 clams, 2,000 worms and 30,000 amphipods! During spring and fall migrations shorebirds gather to feed on this wealth of invertebrates.

Grays Harbor estuary on Washington's coast is one of six major estuary systems on the entire Pacific Coast. Here the freswater of the Chahalis, Humptulips, Hoquiam, Elk and Johns Rivers combine with the salt water of the Pacific Ocean. The estuary's 94 square miles of mudflats, saltmarshes and open water provide critical habitat for a variety of wildlife and fish including up to a million shorebirds during spring migration. The estuary is located in the Chahalis River Watershed, which is the second largest watershed in Washington.

Grays Harbor National Wildlife Refuge (NWR), located along the northern edge of Grays Harbor estuary, occupies only two percent of the intertidal habitat, but hosts up to 50 percent of the migrating shorebids in spring.

The unique conditions at the Refuge make it a focal area for shorebirds during migration. The mudflats on the Refuge are the last in Grays Harbor to be flooded at high tide and the first areas to be explosed as the tide recedes. Shorebirds take advantage of the available mudflats as long as possible in order to obtain a constant supply of food needed for migration. They peck and probe in the mud almost continuously. During high tide, shorebirds rest on high salt marsh or islands above high water. As the tide recedes, the birds follow the water, spreading out over the vast areas of mudflats as they continue to feed.