[SBB] possibly probable semipalm sandpiper at Shoreline

This morning at the west end of Shoreline Park in Mtn View there were probably thousands of Western Sandpipers. They were scattered around in groups in the forebay, the slough, and on the island in the tidal basin. From the paved path on the W side of the forebay I noticed one bird that stood out from all the Westerns in various plumages. It was very slightly smaller with a bill that looked just right for a Semipalmated Sandpiper, clearly shorter and straighter than on all the Westerns. The back was uniform brownish with no rufous. There was a buffish wash across the upper breast. The bird was less bulky appearing than a Western. Black legs, of course. The only feature not well seen was the facial pattern, however the bird was moving around more quickly than the others. After a while an excited Killdeer caused the whole group to flush and I walked on to the other side of the forebay. The flock had already returned and I was able to find the bird again noting the same features. They again all flew off, this time to the island in the tidal basin to join hundreds of other Westerns already there. 4WIW.