Had to go for this one. Two rare terns and a back-up tern.
On arrival picked up the three terns on the north basin and, amazingly, they were all together so it was possible to all three in a scope at the same time.
The terns were (in order of rarity): American Black Tern, White-winged Black Tern and Black Tern. They were quite easy to distinguish.
a. The American Black Tern was a distinctly darker bird to the Black Tern with the upper wing being extremely dark with a obvious dark band on the leading edge to the wing. Also there was distinctive dark smudge on the flanks. At rest it was more difficult to discern from the Black Tern but still looked darker. It appeared to be a slightly bigger bird but that may have been to its wings which looked longer.
b. The Black Tern could be picked out basically being lighter than the American Black Tern without the smudge on the flanks.
c. The White-winged Black Tern was a disinctly larger and paler bird with silvery upper wings, whitish tail and lacking the conspicuous mark on the neck which the Black Tern has. Also the brownish mantle was quite obvious.