We're entering a time period when both our local breeding Eastern Willets and migrant Western Willets can be seen side-by-side, which makes for a great opportunity for study. These are not yet considered full species, but they are distinct in shape, plumage, and distribution, so don't be surprised if one day soon we have two Willet check boxes on our lists.
The photo above shows two birds (July 9, 2012, Stone Harbor, NJ) that are quite similar but subtly different. Notice how the left hand bird, a Western Willet, is:
3. Proportionately longer legged.
4. Longer billed and thinner billed, with a thinner tip.