These two long-legged shorebirds can be easy to confuse with each other, and with at least one other species.
Lesser Yellowlegs are Killdeer-sized sandpipers, but with long legs that can sometimes be covered in mud. The is bill slightly longer than its head. Most winter in the Tropics. In April they begin to show back up in small flocks as the fly around the Gulf of Mexico, not across it. A small flock of feeding yellowlegs almost always Lesser and not Greater Yellowlegs. The call is usually one or sometimes more “teewee” sounds, usually of the same pitch.
Greater Yellowlegs are closer in size to the Willet, but isolated birds can be difficult to identify based on size. Greater Yellowlegs have a bill that is about twice the length of its head, and sometimes shows a slight upward curve. It tends to be more heavily barred than the lesser and tends to be loner. The call of the Greater is much stronger than the Lesser, usually 3 or more descending notes.
All of the following images have the Lesser Yellowlegs on the left, and the greater on the right.
There is one exception. One of the image pairs has a Solitary Sandpiper instead of the Greater Yellowlegs. See if you can spot it.
The head shapes of these two birds are quite different, adding to the possible confusion in identification. The difference is just from different postures.
All images © Greg Lavaty.