Whilst Royal Tern was the dominant species in terms of numbers during my November visit to the Gulf Coast, some other species were present too - Sandwich Tern and Forster's Tern. They were most transient than the larger species, seemingly dipping in and out only briefly, particularly Forster's Tern. I managed a few frames of both in overcast conditions one morning.
The American form of Sandwich Tern, known by some authorities as Cabot's Tern, is a breeder on the atlantic coasts of the USA, and 90% of its population winters in the Gulf. Although the european version has made it across, the overwhelmingly likelihood is Cabot's - and they can be separated by better birders than I on a more robust bill structure and on moult timing at certain times of year. Sound tough? You bet, but below are two images of Cabot's from St Pete Beach.
Below are two images of Forster's Tern, which is a much smaller bird. I wasn't certain I'd ever seen one before as I didn't know what it was when it landed in amongst the throng. It didn't stay for very long unfortunately and I would very much have liked longer with it as it seemed very characterful. I'm racking up a good list of terns now, what with all the species in Dubai last year and then various odd birds on my travels, including a Black-naped Tern from Thailand identified from a nearly 20 year old 35mm slide!