Terns are a challenging and rewarding bird to shoot. Around here we have mostly Forster's Terns (see the photo). Find a deep enough body of water with fish, and you'll see these guys dive-bombing the water, then emerging with a fish in their beaks.
Getting a good photo of these guys takes some work. They're fast - really fast. And they don't fly in paths easily tracked by your camera (remember, you want it in focus, not just in the frame).
My solution is to catch them hovering over a target (shown), or emerging from the water. Or if you're really lucky, you may find one feeding its chick. This should get you a nice shot.
If you want a really good shot, the shutter needs to be fast enough to freeze the motion. This shot was taken at 1/1000" but I recommend a speed of 1/2000 or faster. So raise the ISO as necessary and shoot in Tv.
The reward is the wonderful detail on the wing and tail feathers as well as the beak and talons. For extra credit, try to get a reflection off the eye. Good luck!
You can find product designs based on Terns and other birds in my zazzle store. If you have a product in mind but don't see it, just hit the Contact Me! link on the front page and maybe I'll design it for you!