Sandpipers and Dunlins

I'm about exhausting my shorebird knowledge here, but the birds with the black bellies are the dunlins.  They are a variation of sandpiper, which has the white belly.  Gray's Harbor NWR gets an enormous volume of shorebirds, but not a large variety.  The count peaks in the upper 40,000s, with mainly sandpiper and dunlin and two other similar species. The birds arrive from San Francisco and they each stop over for up to four days and then continue their migration up to Alaska. (This is what I learned from the birders as we waited for the tide to recede.)