Wellington - The population of New Zealand's kakapo - the world's rarest parrot and critically endangered - has increased by seven after the first chicks for three years hatched on tiny Codfish Island.

But that merely brought the number of the nocturnal, flightless birds to 92 and the vulnerable chicks were transferred this week to a specialist unit where they will be hand-reared to ensure their survival until they can be returned to the wild.

Kakapos, once common throughout the country but prone to attack by predators, were thought to be extinct until 1976 when a small population was discovered on Stewart Island and a breeding programme started.

It is a tortuous process as kakapos do not produce eggs annually, breeding only every two to four years. Males do not start mating until they are about five years old and until this year females were not observed to start breeding until they were nine.

But two of the younger females aged six bred for the first time this year, and significantly there was 100 per cent fertility of the eggs laid.

Only 20 chicks have been hand-raised since the parrots were rediscovered more than 30 years ago. This year's hatchings were the first since 2005 when four chicks were produced with an overall fertility rate of 58 per cent.

This year's chicks had to be moved because kakapo mothers depend on ripe berries from native rimu trees to feed their young, but the fruit on Codfish Island did not ripen in the summer which has just ended.

Kakapos are not only the world's rarest parrots, they are the heaviest, with full-grown males weighing up to 2.2 kilograms and females 1.4 kilograms.

Their size and inability to fly is believed to have contributed to their near-extinction at the hand of predators.

The parrots sleep all day, usually in hollows in the ground, and spend the nights on solitary meanders through the bush feeding on seeds, roots and berries.

Survival of the species has depended on Department of Conservation staff and volunteers who have identified all the birds and keep nightly vigils over their nests when eggs have been laid.

They camp near the nests to watch that the females incubate properly, often covering the eggs with heat pads when the mothers go foraging for food.

The kakapo recovery programme has attracted international attention and because the parrots are found only on offshore islands which are predator-free, a website has been established to tell the world about it at www. kakapo. org. nz (dpa)