Hawaiian Petrel. Photo: Natural History Library/Alamy

No one thought that two of Hawaii’s most endangered seabird species still lived on Oahu, the archipelago’s most populous island. Once widespread during the breeding season, the black-capped, white-bellied birds—Newell’s Shearwaters and Hawaiian Petrels—abandoned the island centuries ago, scientists thought, even before modern threats decimated their populations on other Hawaiian Islands.

However, over the last 30 years, a few dozen dazed or dead petrels and shearwaters had been found in Oahu’s urban areas, including Honolulu. One person even made phone recordings of what sounded like the whine of a Newell’s Shearwater in 2006. Those birds could have been lost, off course, or just passing through. Or not. So when Lindsay Young, executive director of the nonprofit Pacific Rim Conservation, started a statewide survey for the two species a few years ago, she decided it wouldn’t hurt to check her home island, just in case.

“We were not really expecting to find anything,” she says, “but no one had ever really looked.”

Young and her team set up acoustic monitoring devices in areas of the island where they thought they would be most likely to find the seabirds. To their surprise, the recordings picked up multiple Newell’s Shearwater and Hawaiian Petrel calls in two remote mountain ranges. The findings were published in January in The Condor: Ornithological Applications.

The birds’ rarity is a relatively recent phenomenon. The fossil record shows that huge colonies of both species once dominated the Hawaiian Islands, Young says. But by the time European colonists arrived in the late 1700s, Native Hawaiians had already hunted them off Oahu. After that, waning populations on the other islands fell prey to introduced predators like feral cats and rats. In later years, light pollution from urban areas drew confused fledglings off course, and power lines became a nighttime flight hazard, further straining the populations.

A survey conducted in the 1990s found 20,000 breeding pairs of Newell’s Shearwaters and 4,500 pairs of Hawaiian Petrels across Hawaii. More recent work suggests shearwater populations have since crashed by 94 percent and petrel populations by 78 percent. Almost all the remaining shearwaters breed on the island of Kauai, while the petrels have small footholds on five islands.

Studying the remaining populations is made all the more difficult by the inhospitable habitats these species prefer. Both shearwaters and petrels burrow in sheer cliffs wrapped in luxuriant vegetation. “In no way had this place evolved for human locomotion,” Paul Kvinta wrote in his Summer 2018 story for Audubon about searching out these species on the island of Kauai. Both species are also nocturnal and most active when humans can’t see them.

To avoid excessive slipping and sliding through the mountain forests, Young decided to use sound recorders to possibly capture the birds’ calls. But first she had to figure out where to put them. She teamed up with ecologist Adam Vorsino with the U.S. Fish and Wildlife Service to model where shearwaters and petrels might be—those areas with both suitable nesting habitat and minimal light pollution. (The darker the better for these light-sensitive species, Vorsino assumed.)

Young hiked to the locations she could and helicoptered in to the hardest-to-reach corners.

Young hiked to the locations she could and helicoptered in to the hardest-to-reach corners identified by Vorsino’s model. “The type of field work required to access these locations is definitely not for everyone,” she says. “I wouldn’t bring my kids on it, I’ll put it that way.”

Over two years, she and her team deployed acoustic monitoring devices in 15 different spots around Oahu. Two devices picked up the braying of Newell’s Shearwaters and the high-pitched coos and squeaks of Hawaiian Petrels in different areas of Mount Kaʻala in the far northwestern corner of the island. Another device recorded more shearwaters at Poamoho, which is about a 40-minute drive northeast of Honolulu.

While Young felt there was at least a chance she'd stumble across some Newell's Shearwaters, hearing Hawaiian Petrels was a true surprise. Confused shearwaters had turned up on Oahu more often than petrels over the years, “and no one had recorded a live one in decades,” Young says. “We were shocked."

The study highlights the advantages of using technology to monitor inaccessible species, says conservation biologist Rachel Buxton of Colorado State University, who has used acoustic monitoring to detect rare, nocturnal seabirds but was not involved with the new study. “It would take hundreds of thousands of hours of people sitting out in the dark to be able to find what they did using the acoustic monitoring devices.”

With the data they have now, scientists can’t determine if the birds are breeding on Oahu or merely visiting. Fortunately, the areas where the birds have been detected so far include swaths of land protected by state and federal governments. Chris Miller, who manages Oahu’s natural area reserves for the Hawaii Department of Land and Natural Resources, says that if there are breeding colonies, he will step up protection efforts, like erecting new fences to keep feral cats and rats out.

Even if the birds aren’t currently breeding on Oahu, they could be “prospecting” the island for future nesting, Young says. If the birds do find suitable nesting areas, it might be possible for scientists to attract more birds to the island and create new breeding colonies in the future, she says.

But establishing a new colony would be many years away, and Miller said he couldn’t comment on whether it would be a possibility. Newell’s Shearwaters and Hawaiian Petrels are listed under the federal Endangered Species Act, which makes taking action around the birds, even positive action, challenging, Young says.

While scientists are still unsure of the way forward, discovering the birds on Oahu remains a hopeful sign. If more islands host shearwater and petrel populations, each species would be less vulnerable to catastrophic events like hurricanes. “Having another population in a safe place could be vitally important,” Young says.

For so long, no one thought Newell’s Shearwaters or Hawaiian Petrels remained on Oahu. Now, it could be the place where these seabirds stage a comeback.