The baking sun and thorny kiawe trees of Kapolei hide dense caches of history, relics from a time when the 'Ewa Plain was a dense forest alive with strange birds now long extinct.

In those years, before the arrival of humans, the amazing moa nalo lumbered through the trees. It was a 3-foot-tall, flightless gooselike duck — the largest of the Native Hawaiian birds. Flightless rails and geese waddled around with it. Overhead flew a sea eagle, owls, crows, a hawk and bats. Finches and other perching birds flitted among the trees.

Most of these birds have been extinct for hundreds of years.

But proof of their existence lies in the bottom of limestone "sinkholes" where they sometimes were trapped and died, leaving their bones and beaks behind. The shells of now-extinct tree snails, and the pollen from the plants that once forested this area are found in sediments with the bones.

The sinkholes are vertical caves in an ancient reef that grew during a period 120,000 years ago when sea levels were much higher. There once were thousands of sinkholes across the 'Ewa Plain — time traps that preserved evidence from Hawai'i's prehistory.

Most have already been filled or covered by development.

Kapolei Property Development, which is proposing a 350-acre light industrial park at Kapolei, plans to preserve a six-acre parcel of undisturbed land that contains several of the sinkholes.

A chain-link fence, stained pale brown with the coral dust of the region, protects the acreage.

"This area hasn't been touched," said Steve Kelly, manager of development for Kapolei Property Development. "It was fenced in the early 1990s by the Estate of James Campbell, and we plan to put a new fence around it. We will be looking to pass on the property to some appropriate entity."

Scientists and community leaders cheer the firm's decision.

"This is a community resource, a place where people can come and learn about the past," said Ati Jeffers-Fabro, an environmental educator who has brought kids to the sinkholes to learn natural history.

"This is all we have left of a unique geological and biological setting in these Islands," said Helen James, a fossil bird expert at the Smithsonian Institution, who with her former husband, Storrs Olson, has taken the lead in identifying the ancient bones and beaks. "For future understanding and research of the Islands' natural history, we should preserve this."

The Conservation Council for Hawai'i is spearheading the effort to ensure protection for the sinkholes. Council executive director Marjorie Ziegler said the organization would like the six-acre plot to be transferred to the Department of Land and Natural Resources, perhaps designated as a state Natural Area Reserve. But if not that organization, some other caretaker should be established, she said.

Key goals are protection, scientific research, public education and the possible reforestation of the area with some of the native plants that the pollen record proves once lived here, Ziegler said.

The first person to find bird bones in sinkholes here was Jennie Peterson, now the environmental education program manager with the Hawai'i Nature Center. During the 1970s, she was an archaeologist with Bishop Museum, studying the area for an environmental impact statement on the then-proposed Deep Draft Harbor.

"I was digging in a large sinkhole when I found bones. They were so big that I thought they were mammal bones, but I knew they couldn't be because they were too light," she said.

'VANISHED FOWL'

No animal known to have lived in Hawai'i could have produced those bones, so she took them to Bishop Museum zoologist Alan Ziegler, Marjorie Ziegler's dad. He recognized they were the same class as extinct birds whose bones had been found in sand dune deposits on Moloka'i, and consulted with Olson, the Smithsonian Institution fossil bird expert, who happened to be conducting research on Maui.

It was a huge bird like nothing alive in the world today.

They called the group "vanished fowl," or moa nalo in Hawaiian. There are examples in the fossil record on all the major islands. The O'ahu moa nalo was given the scientific name of Thambetochen xanion.

A grazing animal, it looked most like a huge goose, but appeared to be most closely related to the dabbling ducks, said James, with the Smithsonian. The moa nalo had lost the ability to fly, and its flight muscles — robust in ducks and geese — were just thin straps across its chest, she said.

Further digging in sinkholes — some of them dry, and some with pools so deep that scuba gear was needed — yielded the bones of dozens of species of flightless birds, land birds, sea birds and raptors. The most common bones came from the ua'u, or Hawaiian dark-rumped petrel. This seabird has never been reported from O'ahu in historic times, but the fossil evidence shows there was an immense number of the birds here at one time.

"There must have been a major colony here. The whole 'Ewa Plain was just covered in them," James said.

When a team of visiting scientists, students and community members explored the six-acre Kapolei site last month, in a few minutes, one collected a handful of bones lying in plain view on the floor of a sinkhole. The collection included wing bones from ua'u and an extinct crow, skulls of ua'u and the beak of a very large, raven-sized extinct crow. The extinct crow has been named Corvus impluviatus.

Today, people think of the arid Kapolei area as former desert, but in pre-human times, it was forested. Researchers have found shells from extinct tree snails, and the pollen from the kinds of vegetation that probably once populated 'Ewa, including pritchardia palms, an acacia that was probably koai'a, and a critically endangered legume called kanaloa.

SURVIVAL PRECARIOUS

Once common, the kanaloa was found growing on Kaho'olawe, but its survival is exceedingly precarious, with just one plant in the wild and one in captivity. There has been no success in getting it to reproduce.

Preliminary dating of the sinkhole material suggests that most of the bird species were in the region for thousands of years, and most disappeared from the area in the years after human contact with the Islands. It is not yet clear what the direct cause was — perhaps humans directly feeding on birds, fire or other kinds of habitat disturbance, human-brought rats that could have both eaten vegetation and bird eggs, or something else.

Michigan State University zoology professor Peggy Ostrom is conducting studies to help answer some of the questions. She said she and her students will attempt to extract proteins from fossils for radio-carbon dating, and to analyze material in the bones to gain information about what the birds ate.

She and James also hope to find clues about the fate of the sinkhole birds.

"I'd be careful about making assumptions. It could have been a number of things," Ostrom said.

There is very little left of the prehistoric life of this region. Almost all the vegetation is modern weeds and hardy introduced trees like kiawe and banyan.

But deep in at least one of the wet sinkholes, in a tiny pool of brackish water, by the illumination from a flashlight, you can see tiny flickers of movement. They are the native anchialine shrimp — living in darkness and among the last survivors of the time when this region was alive with forms of life no living human has ever seen.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.