The recent powerful NW wind system washed up some vegetation on the beach and seems to have caught White-rumps in mid-migration. Before this week I'd never seen a White-rumped Sandpiper at Tiscornia; this week there's been two. They were pretty tame. Most of these photos are barely cropped.
Note the primaries that extend well past the tail and the bill which is much finer than a Dunlin's.
It does have a row of gray winter scapulars coming in amidst the rufous and pale-edged juvenile scapulars.
In spring the bit of pinkish-red at the base of the lower mandible can be a fairly unique mark, in this fall juvie it's limited to the basal couple millimeters of the cutting (tomial) edge of the lower mandible.
This bird is an adult which Tim picked out on the beach from the end of the pier (and note that this nice lighting is from the dune side).
It has slightly more of the pinkish base to the lower mandible but you had to look really hard in perfect light to see it.