Common Tern, juv./1st winter seen on Sanibel FL beach, note the dark carpal bar on the wing.
Here's another view, the solid dark on the head goes all round the nape.
Here it is next to the look-alike Forster's Tern which lacks a carpal bar and the black does not go all around the nape.
Terns are tricky, you must look closely to find something different. Here is the Common Tern standing with some Sandwich Terns, which are larger than it and have yellow tips to their bills.
You never know what you'll find, that's the fun of birding. So, on Dec. 16th, when Don and I were looking over a group of terns on Sanibel Island, FL, one in particular caught our eye. It was a Common Tern, which is listed as "rare to very rare during winter" in A Birder's Guide to Florida.
Looking at Terns means you have to sort out subtle clues. This Common Tern is a juv./1st winter bird. The juvenile plumage is kept from Jul.-Nov. according to our The Stokes Field Guide to the Birds of North America. We noticed the dark line, the carpal bar, on its plumage and also the stand out red base to the bill. This tern was smaller than the Sandwich Terns it was sitting with. It might be confused with a Forster's Tern, which is commonly seen here. The Forster's has a relatively longer and thicker bill, longer legs and a distinctive separate black mask over each eye, not connected by solid black around nape as in other similar terns, like Common Tern. Neither the adult nor juv. Forster's Tern would have the dark carpal bar.
In birding, the way to find rarities, is to look closely, very closely. And also to know what the possible rare birds are that could show up in your area. "Chance favors the prepared mind", as the saying goes.