The Semipalmated Sandpiper is one of the most abundant small sandpipers in North America, but it remains unaccountably rare on the West Coast. These small sandpipers are tricky to identify because of their similarity to the abundant Western Sandpiper (Calidris mauri). I believe the increased number of records of Semipalmated Sandpiper is directly attributable to the advances in field identification over the last 30 years. In the 1970's this bird was considered accidental in California. Today it is regarded as rare but regular.
This individual is in juvenal plumage. The relatively uniform pattern of the upperparts, with only light rufous fringes on scapulars and a relatively scaly overall appearance is quite different from the boldly contrasting rusty scapulars with white mantle "V"s of Western Sandpiper. Semipalmated usually shows a more defined cap and a larger eye than on Western Sandpiper. Lastly the bill averages shorter and blunter in Semipalmated Sandpiper and it lacks the slight kink near the tip of the mandible that gives Western Sandpiper the impression of a decurved tip to the bill.
Bottom shot of the same Semipalmated Sandpiper appears unusually alert. You can see the light buff coloration across the breast, characteristic of the juvenal plumage. This pose clearly shows the slight expansion of the tip of the bill, forming a slight blob at the tip. This is a diagnostic feature of this species which is seldom visible in the field.
This bird was found yesterday by Alvaro Jaramillo and remained associating with the Buff-breasted Sandpiper today.
Olympus D-550z / Nikon FieldScope 3 / 30XWA (hand-held - no adapter).