K_Chan, on that ferry trip, Black-naped Terns composed a large part of the flock that was chasing after the ferry; a couple of Bridled Terns and Common Terns also joined the flock. As the terns moved up & down extremely fast, I only presumed it was a Common Tern when its upper parts and underparts appeared brownish. And Bridled Tern was fairly easily identifiable by the colour of its body.