Much has been written about the identification of Semipalmated and Western Sandpipers. Most of it focuses on minutia, those little differences in plumage that are indeed very helpful in distinguishing these very similar species. The problem is, in order to use all those fine plumage details successfully, a good understanding of how to age shorebirds is required. Rather than rehashing ageing criteria and plumage details, this article will focus on the basics: the more fundamental differences between Semi and Western that should be the foundation upon which any plumage-based identification is made.
So starts Michael O’Brien’s article at birdcapemay.org. With several recent Western Sandpipers being found in Lancaster County at Octorara Lake among flocks of Semipalmated Sandpipers, this is a skill that is very timely to pick up. While reading the rest of the article by following the link above will be the most valuable I will summarize below.
Western Sandpiper (WESA) is slightly larger than Semipalmated Sandpiper (SESA) but the difference is small enough that you would have to see the two of them next to each other to discern the difference.
The bill length difference that is often touted as a identification feature (WESA longer than SESA) can be tricky to see because male WESA and female SESA overlap in bill length.
In overall proportions, WESA tend to look front heavy, with a bigger chest, larger head and thicker neck. This causes them to stand more upright when at rest to balance.
Check out the full birdcapemay.org article for the entire thing as well as some excellent photographs.