Mystery Bird: White-rumped Sandpiper, Calidris fuscicollis

I think I'm going with white-rumped sandpiper for this one. I'm basing that on the appearance of the scapulars, the little bit of "streaked flanks" I think I can see, and "wingtips project beyond tail tip" (Sibley).

SLO: Sandpiper-like object

At least we can be sure about the white rump.

Good call again, John. Specifically, the hint of rufous on the mostly gray scapulars. A Western Sandpiper at this date may well show streaking that far down the flank, although the streaks would probably not be that fine (more chevron-shaped), but a Western would show rufous bases on dark scapulars, contrasting sharply with pale gray wing coverts. Given the posture, I'm not sure I'd feel comfortable saying specifically that the wingtips extend beyond the tail tip -- photos can be surprisingly tricky in that respect. However, it is quite clear that the primaries are very long, which is still supportive of White-rumped, if not quite diagnostic.

With regards to the white rump, you actually can't see that here. The white undertail coverts show up well, and there does appear to be some white above the tail, but all of the peeps (and a lot of other Calidris sandpipers) show white sides to the rump. What makes a White-rumped white-rumped is the way the white extends all the way across the rump -- Curlew Sandpiper and Stilt Sandpiper are the only other (N.Am.) Calidris to show this.