May is when Laughing Gulls start showing up. I don't think I've ever seen one in a big group of Caspian Terns though. Caspian numbers have been way down this year. I don't know if that's because of the long period of N winds in the last third of April or if it relates to decreased salmon fingerling stocking this year.
There's been a ton more Common Terns on the other hand than usually there are in the spring. We had 60-70 of them on the beach the other day. Here's one with a Forster's.
The Common obviously is on the left with darker gray underparts (at least in this pretty close to full breeding bird), a slightly shorter redder bill, noticeably shorter legs, and a tail that falls short of the primary tips instead of extending past. The Forster's also shows lighter primaries, though sometimes this can be affected by the angle of the lighting.
The two can be harder in flight though. Common (above) has darker wings and Forster's much whiter primaries, but Forster's still does have some dark in the trailing edge of the underwing and again lighting can affect the whiteness of the upperwing primaries. I sometimes think that Forster's has longer wings, but this may be optical illusion caused by the whiter primaries showing up better:
Finally a sub-adult Lesser Black-backed Gull who's just the bill short of being full adult; this bird still has some winter head streaking left on the nape.