A Sabine's Gull has been seen, on at least seven different days, from the New Hampshire coast, during the last month. This is a very rare gull for this area, with only 4 previous records. This gull breeds in the high artic and winters in the tropics. In the West, it's regularly seen on migration, especially inland in the fall.
This gull is in it's first summer and easiest to ID when in flight, as it has a very striking wing pattern of a black wedge of outer primaries and their coverts, white mid-wing triangle with its apex at the wrist, and gray inner triangle of secondary coverts. See this nice photo showing the wings, by ace New Hampshire birder, Steve Mirick, click here:
and photo, by him, of the bird standing, click here:
This is a photo, by me, for comparison, of a nonbreeding Bonaparte's Gull, which has a different wing pattern than the Sabine's Gull.
There's no easy way to tell birders how see it, since it wanders around, often alone and not with the other gulls. Here's a map, thanks to Steve, of where it has been sighted, go here:
The beach traffic can be dense, but good luck to birders searching for the gull. The bird could wander to other states as well, so birder's there should be on the lookout.