Willets and yellowlegs are birds similar in appearance, as are the greater and lesser yellow legs.
Greater Yellowlegs
As you have already guessed, “yellow legs” are a distinguishing feature of this long-billed shorebird. It differs from the lesser yellowlegs by it size (14 inches compared to 10 1/2 inches) and by its longer bill (about one and one-half times the width of its head). A white rump and tail is common to both.
These two birds breed in Canada and winter along the southern coast of the United States, and in Mexico and South America. They are seen in Maine as they migrate. This one stopped in Harpswell in October of 2009.
Willets
This shorebird breeds in Maine as well as in Canada, and winters along the shores of the southern United States, the Caribbean, and northern South America. Willets are brown in breeding season and become a lighter brown-gray in the fall and winter. Though of similar size, their gray legs and thicker bills distinguish them from the Greater Yellowlegs.
The willet below was seen in Scarborough Marsh in July of 2010.
Willet in Scarborough Marsh (2010)