Fri-D: Adult Western v. Semipalmated Sandpipers in September

 [Top, adult worn breeding plumage Semipalmated Sandpiper, bottom adult mostly winter plumage Western Sandpiper, both at Stone Harbor Point, NJ on September 10, 2013. Click to enlarge photos.]

First try flicking your eyes back and forth between these two, noticing the Western's bigger head, thicker neck (or neckless look), and subtly more attenuated rear end. That last point was mentioned to me by Kevin Karlson the other night, and I haven't tested it much, but it seems Westerns have a bit of a dip or narrowing at the top rear of the body that Semis don't. This particular Western doesn't have a particularly long, drooping bill, but it is subtly more "Western" than the Semi's.

We can also skip the structure and just note the plumage. Westerns molt earlier than Semis, and by September most are nearly all the way into winter plumage (plain gray upperparts feathers), compared to the Semi which still wears breeding plumage with a few winter feathers coming in. Looking closer for any remaining breeding plumage clues, we see a few of the Western's retained spots on the side/belly, and looking really hard it seems there is a rufous feather still hanging on in the ear covert.