Lesser Yellowlegs have a bill length to head depth ratio of about 1 to 1, (as the bird seen above) it can vary up to about 1-1/4 to 1. The bill usually has an orange to horn colored area at the base of the lower mandible and is thin and straight, unless the bird has just flown into a tree. The nare or nostril opening is very close to the forehead with it almost touching the feathering; compared to the Greater Yellowlegs where the nare is a discernible distance from the forehead. At the other end, the primaries are very long, extending well past the tail and tertials, adding to their slim look.
The call notes of both species are similar, with the Lesser usually giving a two note call and the Greater a 3 note call but both do 2 or 3 and sometimes will give just a continuos stream of notes. I find the distinguishing difference between the two is in the stridency, urgency and stronger quality of the Greater, probably due to its size. Lesser Yellowlegs are more closely related to the Redshanks than to Greater.