The Caspian tern or tarā nui (Hydroprogne caspia) is the largest of the terns. They are about 51 centimetres long and weigh 700 grams. They have a white body and silver-grey wings. In the breeding season their black cap tapers to a fine point above an orange-pink bill. Caspian terns feed by plunging for surface-swimming fish; they also take whitebait, bullies and eels.
These terns are found throughout the temperate world, except for South America. The New Zealand population was estimated at 1,000 breeding pairs in 1992.
The terns breed mainly around the coast, although some nest inland near Lake Rotorua and on river beds in Canterbury. Colonies are usually close to those of other terns or gulls. The birds breed from September to January and lay one to three light-flecked eggs in a shallow scrape on sand. Terns’ chief enemies are black-backed gulls, which eat the eggs and chicks. Chicks fledge at 33–38 days. Caspian terns live about 24 years.
The diminutive fairy tern or tara iti (Sternula nereis) is scarcely larger than a thumb tip when newly hatched, growing to a length of just 25 centimetres and a weight of 70 grams. They have a large white frontal patch with black around the eyes, a fine-tipped yellow-orange bill, and a white body with grey wings and tail. The subspecies Sternula nereis davisae is found only in New Zealand.
They nest from November to January on sand and shell, laying one or two buff eggs with dark brown speckles. The young leave the nest at 23 days. Fairy terns feed on small fish, and can live to 11 years.
In the early 1900s fairy terns were recorded around the North Island coast and on South Island riverbeds, but by the 1980s the number of breeding pairs had dropped to four. The population in 2012 was 30–35 birds (about eight breeding pairs). In 2020 there were fewer than 40 adult fairy terns, but seven chicks had hatched in the 2019–20 breeding season, compared to two in 2018–19. It was still New Zealand’s rarest breeding bird, and its survival remained in doubt.
Cats, rats and weasels eat fairy tern eggs, chicks and adults, and have been responsible for their decline. Storms, high tides, and people riding trail bikes and four-wheel drives also threaten the nests. The New Zealand Department of Conservation has monitored breeding and nests at Waipū, Mangawhai, and South Head, Kaipara Harbour. Their programme has included fencing off predators, relocating nests, educating the public and enforcing laws against the harming of wildlife. Even so, the 8–10 known pairs produce only a handful of fledglings most years, and in some years none at all.