仲の神島の海鳥類

抄録

Nakanokami-shima was designated a national monument in 1972 because of its important for breeding seabirds, though the seabirds of the island were very poorly known. In this paper we present the information on the breeding species and their current status. Observations were made over a ten years period from 1975 to 1984.
Seven species of seabirds were found to breed on the island. All were summer visitors, except for the Brown Booby which may be a resident.
1. Bulwer's Petrel, Bulweria bulwerii. Bulwer's Petrel was not known to breed on the island until we captured and ringed 2 adults on 2 July, 1981, both of which had fully grown brood patches, and 2 more adults on 4 July, 1982, both of which were incubating. In 1982, 1983, and 1984 the petrels bred at the same locations shown in Fig. 1. The petrels bred gathering in small numbers and laid under rock, Sixty-six adults were ringed from 1981 to 1984 and it seems that the breeding population is fewer than 100 birds.
2. White-faced (or Streaked) Shearwater, Calonectris leucomelas. Many nest holes of the White-faced Shearwaters were widely distributed on gentle slopes in grassplateau in the centre of the island. Some birds incubated on bare ground under dwarf 'Gajumaru', Ficus retusa, and others incubated under large rocks. We could not estimate their population.
3. Brown Booby, Sula leucogaster. The main colonies, which were used by almost all the birds, were found annually at areas A, B, C, and D shown in Fig. 2. On Nakanokami-shima Brown Boobies nested on ridges and on steep cliffs. Few birds nested in the rocky zone near the shore. Before noon on 3 July, 1981, a Maritime Safety Agency Helicopter flew over the eastern part of the island, surprising the settled birds and causing them to take flight one after another. It was possible at that time to make a total count and 250 birds were counted. Since the breeding pairs were taking care of chicks at that day, some parents were probably absent offshore. Clearly the population was larger than 250. A chick ringed on 29 June, 1980, at the main colony was recaptured at the same place on 21 August, 1983. The bird was in adult plumage, but was not breeding. This recovery record indicates that non-breeding immatures are also included among those attending the main colonies. Table 1 shows the results of an intensive search for nests during the breeding season of 1984. Nests on inaccessible cliffs in areas A-D were of course omitted. The annual breeding population was estimated approximately as about 200 to 500 birds.
4. Red-footed Booby, Sula sula. This species was not known to breed in Japan before 1975, when we discovered a breeding pair on 27 August, 1975. The parent incubated one egg and its nest was builted on the canopy of dwarf 'Gajumaru' bush in area A in Fig. 2. On 24 June, 1976, 3 adults and 2 chicks were ringed at the same place, and on 30 June, 1977, one downy chick and parent were found again at the same place as in 1975. On 13 July, 1982, two fledglings and one adult were seen on the cliff in area E in Fig. 2.
5. Bridled Tern, Sterna anaethetus. This, species was not known to breed in Japan until we found its breeding at the island on June, 1980. The Bridled Tern settled rocky zone near the shore, placing their eggs in the shelter of rocks. The locations of colonies in 1983, and 1984 are shown in Fig. 3. This tern is apparently an inshore feeder, remaining usually close to the colony. The estimated numbers were 120 birds in area A, and 40 in area B on June 30, 1980, about 1000 in area A, and 100 in area B on 2 July, 1981, and about 650 in area, A, and 50 in area B on 2 July, 1982. Since 1981 the numbers have increased markedly. The birds in attendance at these areas were in adult plumage, however considerable numbers of non-breeding, presumably immature, birds may make up part of these totals.