That's the right tern there Mauser, I've seen experts get the wrong tern at times...I beleive that's a female or as it is scientifically called a ewe tern. When you see them and they fly away of course it's a left tern....
I would agree common tern if it wasn't for the black legs and feet. All my guides tell me that the common tern has red legs and feet and is more pigeon sized. Comparing the size to the ring-billed gulls, it appears larger than a pigeon and the legs look black. None of my books mention young or winter plumage where the legs are black on the common tern.
The Royal Tern on the other hand has black legs and feet, is crow sized (same as the ringed-bills) but has a light pumpkin to yellow colored bill. That bill looks more red to me.
The Caspian tern on the other-other hand has a large thick red-orange bill and black feet. It also has the dark primaries on the undersides like the above. The Caspian is the largest of the terns, reaching the size of say a Herring gull or 21".
So if I had to guess, I would go with Caspian Tern, but naturally size is hard to tell from a photograph...and I am more of a songbird guy myself, though I do need to brush up on the shorebirds! There have been reports of Caspian Terns from the Conjohela Flats recently as well.
but caspians have a slight tuft on the back of their head...royals have a big tuft on the back of their head...
but, from the pics, you may not really see the tuft...which was what i was looking for..and ive seen the wind if its right, lay it down...so i think your right and it is a Caspian...didnt even look at the legs! great eye!