There were a few days this past summer when I repeatedly took the ferry between Cheung Chau and Central. Common Terns were present much of the time. Mind you, they were of the eastern race
longipennis:
https://ebird.org/species/comter2
On those Common Terns, even from a distance, I could detect a clear contrast between the grey wings and the white rump and tail. You will not see such a contrast on a Roseate Tern because the wings of a Roseate are of a much lighter shade of grey.
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Last edited by bittern at 1/10/2021 19:03 ]
