Bonaparte's Gulls are the smallest type of gull commonly found on the east coast (Little Gulls are smaller, but quite rare). During the summer, while adult Bonaparte's are on their breeding grounds in central Canada and Alaska, some immature birds spend their summer days on southern Maine beaches. First-cycle birds (like the two pictured here) have a dirty look, with dark-tipped flight feathers, a solid-black tail band, and either a dark ear spot, or a partial dark hood.
Aside from these plumage details, get to know their behavior. You're unlikely to find these gulls vying for food at an urban dumpster. These small gulls often flock up (mainly with other Bonaparte's) and actively pick food off the surface of the water. Their flight style and calls can be quite similar to that of terns. On more than one occasion, my ears have been momentarily fooled.
For more on Bonaparte's Gulls, visit All About Birds.
Note: Bonaparte's Gulls are the feathered stars of Quiz #108: Bird.