Common tern vs roseate tern

Common tern and roseate terns looks very similar, how can they be separated?
Thanks in advance
So longer tail, different bill and no black on underside of wings.

When the bird is sitting, the best features are the bill, tail length, and overall paleness. Common Terns have a bill that is typically red with a black tip. Roseate Terns typically have a mostly dark bill that is distinctly longer and more dropping than on a Common. The long tail of Roseates becomes obvious, as it extends well past the primary feathers on the wing. On a Common, the tip of the tail and tip of the folded primaries are usually pretty close. Finally, Roseate Terns often look strikingly white/pale, versus the more gray-toned Common.

In flight, the overall coloration/brightness is again a major clue. Common Terns look more grayish on the back and wings as well as on the underside. Roseates will look bright white. Again, the tail length is something to look out for. On the wings, Common Terns show a thick, diffuse dark gray border on the rear edge of the primaries. The wings on a Roseate will show very little dark coloration on these primaries.

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Also worth considering with perched birds - albeit easier with experience - is relative leg length: Roseate looks much longer-legged than Common.

Are you trying to identify them in flight or perched? In breeding season or wintering area? One field mark I have found very useful for wintering birds, that is often visible in photos, is that Roseate Terns have long, totally white tail streamers. Common Terns have shorter outer tail feathers that are grey on the outer web. There is more information here:

I’m trying to id them in flight, as they are mostly seen chasing boats in Hong Kong.

Good luck! The longer you spend observing them, the more likely the differences are to “click”. Look in particular at overall whiteness (upperparts and underparts), tail streamer length, tail streamer colour, underwing pattern, and get familiar with immature and adult plumages. If you have a good camera, take a lot of photos so you can pick out and study details later that you might have missed. Sterna terns are hard, but if you persist you’ll start to see the differences, and even start to be able to pick them out at a distance.

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