Greater and Lesser Yellowlegs
There is one easy way to tell the Greater Yellowlegs (Tringa melanoleuca) from the Lesser Yellowlegs (Tringa flavipes): just find them standing next to each other. Taken at the Yolo Bypass Vic Fazio Preserve near Sacramento.
As you can see, the Greater Yellowlegs, as big as a Willet, is considerably larger than the Lesser, who is more Dowitcher-sized. Differences in posture here exaggerate the size of the Greater, but the more important features shown are the size, shape, and color of the bills. The Lesser YL's bill is black, thin at the base, and about as long as the apparent size of his head. The bill of the Greater YL is gray and thick at the base, and clearly longer than its head. When we see a Yellowlegs by itself we are well served to look carefully at the bill. Voice helps too, as the Greater cries out quite loudly in flight 3-4 times in succession, while Lesser gives a softer 1-2 note cry.