Greater (right) and Lesser (left) Yellowlegs, Jackson Bottom Wetlands, Hillsboro, Oregon on 7 September 2010 by Greg Gillson.
Greater Yellowlegs is a common spring and fall migrant throughout the Pacific NW. It sometimes winters west of the Cascades in fresh water marshes. Lesser Yellowlegs is slightly less common.
Telling Greater from Lesser Yellowlegs is much easier when they are together, as in this recent photo above. When they are alone, or in single-species groups, it is harder to make this identification.
The key for lone birds is a comparison of relative bill length and shape.
The bill of Greater Yellowlegs, below, is rather thick (about as thick as the eye) for about half its length, and then appears to be very slightly upturned. Equally important, the bill length is about one-and-a-half times the length of the head in a straight line with the bill back from the gape, as shown in red:
In contrast, the bill of Lesser Yellowlegs, below, is rather thin and tapering throughout. The total length is just barely more than one head length, as shown by the red lines:
So, a bill length of one-and-a-quarter head lengths is a good rule of thumb for dividing between the species. Of course, there is a bit of variation and some birds might be too close to call.
Speaking of calls, this is when you need to use them. Greater Yellowlegs has a call that is a series of three loud piercing notes: tew, tew, tew.
The diminutive Lesser Yellowlegs has a correspondingly softer voice, and only one or two whistled notes: yip or yew, yew.