Hoshigami Island Streaked Shearwater Breeding Ground

Hoshinokamijima Streaked Shearwater Breeding Ground: An Uninhabited Island in the Oki Archipelago

About 1.2 kilometres off the rocky northwest coast of Nishi-no-shima, in the Sea of Japan, a 74-metre-high islet of just three hectares rises from the water. Hoshinokamijima (also read Hoshinokanjima in the official register) has no permanent residents, no jetty, and no path. Yet for at least several centuries, tens of thousands of Streaked Shearwaters have returned here every spring to dig burrows into its soil, raise a single chick, and depart again before winter. In May 1938, the Japanese government designated the entire island a National Natural Monument, recognising it as one of the country's most significant seabird colonies.

The island sits within Daisen-Oki National Park and inside the boundaries of the Oki Islands UNESCO Global Geopark. Visitors to Nishi-no-shima can glimpse it across the strait, but landing is neither encouraged nor practical. The story of Hoshinokamijima is best understood from a distance: a story of one extraordinary seabird, an act of early conservation legislation, and a small island that ships once worshipped on their way home.

A Speck of Land in the Sea of Japan

Hoshinokamijima belongs administratively to Nishi-no-shima Town in the Oki District of Shimane Prefecture, lying off the cape known as Nishise-zaki. Its perimeter measures roughly 0.7 kilometres and the highest point reaches 74 metres above sea level. Sheer cliffs ring most of the coast, with seabird-friendly slopes of soil and low vegetation on the upper plateau where shearwaters can excavate their nests.

Before the era of steamships, Hoshinokamijima held a different kind of significance. During the late Edo and Meiji periods, when the kitamaebune cargo vessels worked the long sea route between Osaka and Hokkaido, captains passing through Oki waters are said to have prayed to the island for safe passage. Local tradition also remembers Hoshinokamijima as a place of rain-prayer worship. Whether these beliefs explain the island's name (星神, "star deity") or arose from a name already in use is no longer clear. What survived into the twentieth century was the sense that this small place was different from the rocks around it.

In the early Shōwa period, that sense was formalised in a new vocabulary: the island became a natural monument, protected not for its religious associations but for the seabirds that crowded its slopes each summer.

The Streaked Shearwater: An Ocean Bird Built for Long Distances

The Streaked Shearwater (Calonectris leucomelas) is the largest member of the shearwater family found in Japan and the most commonly encountered from late spring through autumn. Adults measure about 49 centimetres from beak to tail and stretch a wingspan close to 120 centimetres. The upper body is greyish-brown with a distinctive salt-and-pepper mottling on the head, while the underwing is white. The bird's Japanese name, ōmizunagidori, refers to the way it skims low over the waves with stiff, outstretched wings, as if slicing the water with a naginata polearm.

At sea, these birds are masters of efficient flight. They exploit ocean winds and sea breezes to glide for long distances with very few wingbeats. Tracking studies on birds from breeding colonies in Iwate Prefecture have shown that foraging trips during chick-rearing can range from same-day excursions to expeditions lasting up to two weeks, with birds sometimes flying as far north as the waters off Kushiro in Hokkaido in search of small schooling fish. During the seasonal migration south, individuals have been recorded covering around 1,000 kilometres in a single day.

On land, the same birds are surprisingly clumsy. Their legs are set far back on the body for swimming, which makes walking difficult. Researchers visiting colonies have noted that adults can sometimes be picked up by hand. This vulnerability is part of why undisturbed islands such as Hoshinokamijima matter: the birds need a place to nest where they cannot easily be caught by ground predators.

A Year on the Island: The Breeding Cycle

The shearwaters arrive at Hoshinokamijima in March, returning from their wintering grounds in northern Australia, Papua New Guinea, and the seas around the Philippines. The pre-laying period stretches across three months, during which pairs reunite, refurbish old burrows or excavate new ones, and feed at sea. Each burrow runs roughly 15 to 20 centimetres wide at the entrance and extends more than a metre horizontally into the soil. At the back, the female lays a single white egg in late June, usually on a small bed of leaves and twigs.

Both parents take turns incubating, with the off-duty bird heading out to sea to feed. After hatching, the chick is left alone in the burrow during the day while both adults travel long distances to bring back fish, regurgitating their catch at the nest after dark. By October, the chick weighs roughly one and a half times as much as either parent, a reserve of fat that will sustain it through the most dangerous weeks of its life. The adults then leave the island first. For about three weeks the abandoned chick continues to grow, drawing on its accumulated reserves, until it eventually emerges and launches itself toward the sea in late November or early December.

By midwinter the island falls silent. The burrows remain, hidden under grasses and earth, waiting for their occupants to return the following spring.

Why a Natural Monument? The 1938 Designation in Context

Japan's modern system for protecting natural heritage was established under the 1919 Law for the Preservation of Historic Sites, Places of Scenic Beauty, and Natural Monuments. Streaked Shearwater colonies were among the earliest beneficiaries. The first designation, Kanmuri-jima off Kyoto, came in 1924. Watashima-Ōshima in Hokkaido followed in 1928. Sankan-jima in Iwate was added in 1935. Hoshinokamijima became the fourth designated colony on 30 May 1938.

The criterion cited in the registry is criterion (ii): not a uniquely Japanese species, but a notable Japanese animal whose preservation, together with its habitat, is necessary. Two years later, in 1940, a second Oki island, Okinoshima off Dōgo, joined the list. The most recent addition came in 1972 with Awashima in Niigata. The fact that two of Japan's six nationally designated shearwater colonies lie within the Oki Archipelago says a great deal about the importance of this remote group of islands for breeding seabirds.

The designation does not impose a fixed boundary, fence, or visitor centre. It does, however, prohibit unauthorised modification of the island and any activity that would harm the breeding population. Authority for management was transferred to Nishi-no-shima Town in July of the same year.

The Wider Seabird Archipelago

Hoshinokamijima is not the only seabird island in the Oki group. Ōhakajima in the Chibu village waters is a prefecturally designated breeding ground, while Okinoshima off Dōgo carries the same national designation as Hoshinokamijima. Streaked Shearwaters also breed on Hakushima, Matsushima, Futatsumatajima, and Ōmorijima. The cliffs and offshore rocks around Nishi-no-shima support additional seabirds that share Hoshinokamijima's habitat, including Black-tailed Gulls, the threatened Japanese Murrelet, and Tristram's Storm-petrel.

The Oki Islands sit at a meeting point of warm and cold currents, with the Tsushima Current sweeping up from the south and the Liman Current bringing colder water from the north. This convergence creates rich fishing grounds for sardines, mackerel, and squid, which in turn feed the seabird colonies. The same productivity that has supported Oki's human fishing communities for a thousand years also supports the shearwaters that fly hundreds of kilometres a day to provision their chicks.

Recent decades have brought new pressures. Some Oki colonies have seen declines linked to sport fishing visits during the breeding season, and introduced rats have devastated shearwater chicks at other Japanese sites. The protected status of Hoshinokamijima is one part of a larger effort to keep these threats at bay.

Experiencing the Story from Nishi-no-shima

Hoshinokamijima itself is closed to casual landing, and there is no scheduled boat service to the island. The experience of this Natural Monument is therefore an indirect one, built up from the surrounding landscape and the seabirds visible from accessible parts of Nishi-no-shima.

The most direct way to gain a sense of the seabird environment is to take the Kuniga Coast sightseeing cruise, which departs from the Nishi-no-shima ports. The cruise passes the 257-metre Matengai cliffs (Japan's tallest sea cliffs of their kind), the natural arch of Tsūtenkyō, and the Akekure sea cave. Shearwaters and gulls range along this entire coastline during the breeding season, and the same waters where they feed are visible from the deck. The cliff-top walking trail at the top of Matengai also offers a chance to scan offshore for birds.

The Oki Islands UNESCO Global Geopark visitor centres on Nishi-no-shima and at Saigō Port on Dōgo provide further context, including information about the seabird colonies and the geology that made them possible. Spring and summer are the seasons when shearwaters are present in Oki waters, with peak activity from May through September.