KEY POINTS:
Volunteers battling to save New Zealand's rarest native bird have discovered a tiny fairy tern - one of only about 40 of its kind in the world - crushed to death on its nest.
Department of Conservation staff believe a careless beachgoer crossed a tape fence and stepped on the chick at Mangawhai Spit north of Auckland, days before it could have flown the nest.
The death is a blow for volunteers and wardens trying to save the birds, whose camouflaged chicks are almost impossible to see in poor light.
There are now only four fairy tern chicks left from about 12 breeding pairs - two chicks at Mangawhai and two at Pakiri.
Warden John Winters, who guards fairy tern nests at Mangawhai Spit, said it appeared someone had crossed into the nesting area and walked on a chick last Tuesday night after he left work for the evening.
A trail of footprints suggested the person trampled the chick, probably by accident, despite a tape barrier and signs warning people to keep out.
Distressed volunteers discovered the dead chick the next morning when they arrived to guard the nest.
The New Zealand fairy tern has unique DNA and is the most endangered bird in the country. Breeding pairs have risen to about 12 from three in 1983. But conservationists are at a loss to explain why almost half the population is not trying to breed.
Earlier in the breeding season, three potential nests at Waipu in Northland were lost when cats ate eggs from the nests.
DoC area spokesman Reuben Williams said the crushed chick had been sent to Auckland Zoo for an autopsy. The Herald understands the chick had internal injuries and bruising consistent with being stood on.
Mr Winters said people walked into breeding areas only a couple of times a season. The cordoned areas were very small compared with the size of Mangawhai Spit, so there were plenty of other places to walk.
The light-coloured chicks were hard to see in their shallow, sandy nests. Their instinct if they sensed danger was to freeze and rely on their excellent camouflage to save them.
Mr Winters said if the chick had lived a day or two longer, it would have been old enough fly away when danger approached.
Mr Williams said there were up to five pairs of fairy terns trying to "settle down and have babies" on Mangawhai Spit.
"We're doing all we can [to protect them] but to a certain extent we rely on the public to make themselves aware and give the birds a bit of space.
"While the area is set aside as a wildlife reserve, it's still very much a public space."
As well as fencing off nesting areas, DoC traps predators and hires full-time wardens during the breeding season.
Mr Williams said the loss of a chick was a setback.
* NZ fairy terns
Our rarest and most endangered bird. DNA unique to New Zealand.
Weigh an average of 70g and are roughly 250mm long. Baby fairy terns are about the size of a bumblebee.
Once widespread around the North Island coast and east coast of the South Island. Now live only in the lower half of the Northland peninsula.
Breed in Mangawhai, Pakiri, Waipu and the Papakanui Spit. Prefer nesting in open areas with shell cover, on low-lying sand spits near estuaries.
In 1983 only three breeding pairs remained. Now thought to be about 12 breeding pairs.
Adults can be picked out in breeding season by their black caps and soft grey feathers. When not breeding the crown fades to mottled black and white.
The oldest known fairy tern was 14. Most live to six or seven years.
