BirdingNZ.net

I would say that this is an immature White-winged Black Tern. The long wings relative to the tail, the distinctive head pattern (dark ear covers, cap, and upper nape), and the dull reddish-pink leg suggests that this is either one of the marsh terns or Black-fronted Tern. The square tail, accentuated by the round tips to the outer tail feathers, rules out Black-fronted Tern, while the white rump, which the bird is preening, rules out Black Tern and Whiskered Tern. The dark primaries would also run out Black-fronted Tern. (The little that one can see of the base of the bill looks black, as opposed to dark red, also suggesting White-winged Black Tern rather than Black-fronted Tern.) The leading wing coverts look darker grey than those behind them, suggesting an immature, as do the dark tips to some of the lesser and greater coverts (although it is difficult to tell if this is shadow; I don't think so).

Peter Frost

immature fairy tern

From Noel Knight,
My guess is White-Winged Black Tern

White-winged Black Tern

White-winged Black Tern

White-winged Black Tern - legs look too long (& wrong colour) for Black-fronted Tern, and black pattern on head seems right for WWBT.

white-winged black tern

Juvenile Arctic Tern (based on pale reddish colour of the leg)

It's a stinker, this one.
Red short legs. Arctic tern.
White ear coverts. God knows what.
White-Fronted. Black inner primary webs and plain grey mantle and wings all correct. Legs right colour but too short.
Black-fronted Tern. No. Tail and primaries too dark. Legs wrong colour.
Little Tern. No. Mantle and wings too plain.

Tried to turn this tern into everything. But it keeps coming back to a bog-standard White-Fronted. Still don't like the
short legs and particularly not the white ear coverts. So I'll go for it anyway, and look a total idiot. So much looks wrong for the species, but the wings/tail don't fit anything else.

White-Fronted.

I'll go for white-winged black tern (adult non-breeding), based on head pattern, light coloured plumage and reddish legs