Telling the yellowlegs apart is difficult, but this one at Heislerville last Saturday stopped me right away as a Lesser. Here are two views of the same bird, which has a short bill even for this species, about equal to the width of the head. So this one is a "gimmee:" Lesser Yellowlegs, almost certainly a male because of the short bill (female shorebird bills are longer on average than males.) I generally say that if the bill is 1.2X the width of the head or less, it points to Lesser, 1.3X or greater points to Greater. With practice, this is the most helpful way, next to voice, to tell the yellowlegs apart.
The illustrates an important point: Within species, there is variation. There are short- and longer-billed Lesser Yellowlegs (and Greater), and short- and longer-billed Semipalmated Sandpipers and most other shorebirds, too. Birds of the same species of course can vary in plumage, too. That's why it's always useful to ask questions like, "What's THIS Lesser Yellowlegs look like?"