Here we have a group of birds on a sandbar or mudflat. For the birds that aren’t preening, we can see that their bills are all relatively sharp and their bodies are rather sleek, so they are terns, not gulls. As with other birds, size is often a great place to start, rather than focusing on plumage patterns. I see three size classes here. The third bird from the left and the one in the center are clearly larger than all the others. Their bill colors are slightly different, with the one in center having a yellowish bill and the one on the left being more orangey. Another difference is that although both of the large birds have a white cap with a ‘Friar Tuck’ haircut, the one in center has a more extensive white cap. Despite these differences, these are both Royal Terns, with the one in center being an immature bird. Royal Terns can be most readily confused with Caspian Terns, because they are similarly sized and both species have black caps in breeding plumage, but Caspian Terns maintain a more extensive black cap into autumn and would have a brighter red bill with a smudgy tip.
Now let’s move to the other end of the size spectrum. Three birds are clearly smaller than the rest, and all three have dark wings, short legs, a patterned head, and an elongated shape due to their long wings. These are all Black Terns in non-breeding plumage.
That leaves us with six more birds (one of which is obscured by the left-most Royal Tern, and the rightmost bird being partially cut off). They are all similarly sized, but are there any features to distinguish them? Three birds have dark bands on the ‘shoulders’ of their wings, while the other two that are in full view have completely plain silvery wings. These features are good enough to distinguish Common Tern and Forster’s Tern in their non-breeding plumage. The three birds with the dark carpal bar are Common Terns, while the two birds with the unmarked silvery wings are Forster’s Terns. Confirming these identifications, the one Common Tern whose head is completely visible has a complete black cap, while the Forster’s Terns only have a black eye patch. These two species have identical black caps in breeding plumage, but the Forster’s Terns lose that cap earlier than Common Terns in fall.
There you have it: eleven birds consisting of four tern species. I love sorting through tern flocks.