Work is complete on what could be the largest cat-proof fence — five miles in length — in the United States.

The fence in Hawaii Volcanoes National Park is designed to protect the federally endangered ‘ua‘u, or Hawaiian petrel, from the birds’ primary threat: feral cats.

The seafaring ‘ua‘u nests in deep lava rock burrows on the rugged high-altitude slopes of Mauna Loa, and, despite the remote location, are not safe from cats. The specifically designed barrier is more than six feet high, and has a curved top section that prevents cats from climbing over it.

Construction began in 2013, and was limited to January through May to avoid disturbing nesting birds. The seabirds spend most of their lives at sea, and come to land only during breeding season. ‘Ua‘u return to the park briefly in April to prepare nest sites, and return in early June to lay a single egg.

The fluffy chicks hatch in August and remain in their burrows until November when they fledge or take their first flight out to sea. Adults, eggs and chicks are extremely vulnerable to predators throughout the long breeding season as all activity occurs on the ground.

The fence now protects more than 600 acres of ‘ua‘u nesting habitat on Mauna Loa.

Construction crews worked and camped at elevations between 8,000 and 10,000 feet, in steep and loose lava rock terrain, and in weather that ranged from hail, and high wind, to extreme heat. The site is very remote and all materials, gear and staff had to be flown in and out.

The endangered Hawaiian petrels are more typically seen on neighbor islands. The species is very rare on Hawaii Island, with just 75 nesting pairs in Hawaii Volcanoes National Park, and another small population on the slopes of Kohala.

Both parents take turns incubating a single egg, and later, feeding the chick. They fly from high atop Mauna Loa to forage in the Pacific Ocean, ranging as far north as Washington State before returning to the nest to feed their chick.