Least Terns are the smallest of Maine's five species of breeding terns. They typically nest on beaches, and face many of the same challenges as Piping Plovers. Indeed, both species are listed as endangered by the state of Maine. I know of two nesting colonies here in York County, Maine: one at the mouth of the Little River, which separates Wells and Kennebunk, and another on Stratton Island (~3 miles east of Old Orchard Beach).
This juvenile Least Tern is doing his/her best to blend in with the sand and rocks, while an alert adult looks on.
Assuming you get more than a fly-by glance, identifying a Least Tern isn't too tricky. Breeding birds have a white forehead patch and yellow bill with a black tip (the black tip is hardly noticeable in the above photo). Many birders also rely on their loud calls for identification confirmation. (Listen to the following embedded sample.)