The whiskered tern (Chlidonias hybrida) to be exact. It is an agile and nimble bird, and a pleasure to watch it swoop and dive for fish, in MaiPo marshes in mid May.
In fact the genus name is from Ancient Greek khelidonios, which means "swallow-like"; a very apt term
Here is one catching a fish, after a short, swooping dive.
A more slender bird than many other tern species, this marsh-dwelling bird has two distinct plumages; one for breeding or summer plumage, when the whiskered tern has a dark grey body, white cheeks and a black crown, whilst in winter, the plumage turns white on the underside and pale grey on the upperparts. The bill and legs of the whiskered tern are red, a striking feature that allows this species to be distinguished from other terns.