Terns belong to the Sternidae family. They are distinguishable from gulls by a fork-like tail, which has led to their popular name of sea swallow. At breeding time their plumage changes and their cap becomes deep black. After breeding the black fades and the colours of the bill become less intense.
Six species of tern breed in New Zealand, while several others are regular or rare visitors. Four tropical or subtropical members of the family, called noddies, breed on the northern Kermadec Islands.
The black-fronted tern or tarapirohe (Chlidonias albostriatus) occurs only in New Zealand. It is the one tern that only breeds inland.
Black-fronted terns are small – 29 centimetres long and 80 grams in weight. They have white cheek bands, a red bill and white body with grey wings. Their forked tail is pale grey, and their rump is white.
They breed from September to January, laying one to four dark eggs with brown blotches in a shallow scrape on shingle. Like all of New Zealand’s terns, both sexes incubate the eggs. The young fledge at 30 days.
Known as ploughboys or the ploughman’s friend, black-fronted terns feed on grubs and worms from freshly dug earth. Along rivers and streams they eat mayflies, stoneflies, skinks and small fish. Once summer is over, the terns leave their nesting grounds for coastal and inshore areas from Stewart Island to the southern North Island, where they feed mainly on crustaceans in the plankton.
Some black-fronted terns live along the braided rivers of the South Island, and like other birds there, are under threat from hydroelectric development, predators and rampant weeds, which smother nest sites. Around 60% of New Zealand’s 5,000-strong population lives in the upper Waitaki River basin, where in 1991 a recovery project was launched to halt the birds’ decline.