Stilt Sandpipers are moving through Cape May right now and can be found most days in the freshwater marshes at the southern tip of the peninsula - especially the Meadows and Cape May Point State Park. While this species is highly distinctive and quite striking in breeding plumage, I still think of it as a "tweener" shorebird - a species that can be easily mistaken at a glance for a few other species (Pectoral Sandpiper is another such "tweener" shorebird, easily mistaken for Least Sandpiper, Ruff, and others depending on the context, distance, etc). In this case, flocks of yellowlegs and dowitchers can easily provide "cover" for Stilt Sandpipers. With a careful look at the structure, colors, and size of Stilts, they become much easier to pick out of flocks.
July-August is when adult Stilt Sandpipers molt from their dark-barred alternate (breeding) plumage into the duller basic (winter) plumage. This flock of adults still shows plenty of heavy barring across on the underparts; among our American shorebirds, this pattern is unique to Stilt Sandpiper.
The slim shape and gray and white colors may recall the pattern of yellowlegs, but these Stilt Sandpipers in their unbarred, basic plumage are more compact in wing shape and overall length. The underwing pattern, a white stripe on the underwing coverts isolated in gray, helps set the species apart from yellowlegs and dowitchers which have more evenly patterned underwings.
This photo of a Stilt Sandpiper (left) with a Lesser Yellowlegs (out of focus at right) gives an idea of the relative shape and size of these two species. The yellowlegs is relatively leggy and shorter-billed than the sandpiper. You can also see the underwing of the yellowlegs, which is more uniformly pale/ evenly barred than a Stilt Sandpiper.
Stilt Sandpipers often feed in deeper water with Short-billed Dowitchers, and so you often see the two species together in flight. Note the slim shape of the Stilt Sandpiper's body and wings, the leg projection beyond the tail, and the slightly droopy bill. Compare the isolated white stripe in the middle of the underwing of the Stilt Sandpiper to the evenly barred underwing of the dowitcher.
Of course, dowitchers (of both species) show an obvious white stripe running up the back, which Stilt Sandpipers lack. Here, a Stilt Sandpiper (at left) shows off its pale hind end and dark back, strikingly different from the high contrast backs of the four Short-billed Dowitchers.