ajohn

It's a Common Greenshank on the right and a Marsh Sandpiper on the left.
The two a fairly similar, but notice that the Greenshank is larger with a thicker bill (Marsh Sandpiper has a very thin, needle-like bill). If you look closely you will also see a difference in the upperparts, especially tertials - Greenshank has white spots ('notches') along the edges whereas Marsh Sandpiper has darker markings across the feather.