Wildlife biologists say they've discovered an unexpectedly large population of the endangered Hawaiian petrel, or 'ua'u, in the remote mountains of Lana'i.

The state Department of Land and Natural Resources said Friday it deployed a team of biologists to the island to learn more about the birds and their conservation needs after the discovery was made last year.

"The Lana'i Hale watershed had not been surveyed for petrels since the 1980s, so we didn't know what to expect," said Scott Fretz, DLNR wildlife program manager.

"We assumed there would be few, if any, birds remaining on Lana'i, but once we started the surveys we immediately realized that we had found something special."

"We don't yet know the total number of birds on Lana'i, but there appear to be hundreds, if not more, which would make this one of the biggest populations known in the state," Fretz said.

"This discovery indicates that the population there has grown significantly in the last 20 years."

Once common throughout the Islands, the petrels were decimated by such predators as cats, rats and barn owls, as well as the loss of native habitats that the birds depend on for nesting. They had all but disappeared from Lana'i by the 1980s.

The birds spend most of their lives at sea, returning to land only part of the year to breed and to fledge their young.

Even then, they only return to the upland nesting areas after dark and then fly to sea to feed before dawn.

To study the elusive species, biologists employ special methods, such as thermal imagers, night vision technology and marine radar to gather the information needed to develop conservation programs to protect the birds, the DLNR said.

Allan Smith, DLNR interim chairman, said the state is working with landowner Castle & Cooke to develop new phases of the work that will protect larger areas of the watershed.