A new attempt to estimate the number of nesting petrel, Oceanodroma leucorhoa leucorhoa, and wildlife of the Daikoku Island, Hokkaido
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概要
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An ecological survey of Daikoku-jima Island was made July 4-8, 1972. It is a 1, 074, 000 km2 islet (six kilometers round, one hundred meters high) located in eastern Hokkaido about 12 kilometer off Akkeshi Pay (Fig. 1), and has the only known breeding colony of the Leach's Fork-tailed Petrel in Japan. There are several fishermen in the northern tip in summer and two lighthouse keepers in the southern end of the islet. The southwest corner of the island (119. 937 m2, 11% of the total area) is conserved as a seabird breeding sanctuary (ig.3).Thirty-six species of birds and three species of mammals were observed on and around the islet (Table 1). Most of the islet is covered with herbaceous plants(Fig. 5), such as Artemisia montana, Urtica platyphylla, Ligularia hodgsonii, Calamagrostis langsdorffii, Sasa nipponica, Polygonum sachalinense etc., and a poor broad-leaved woods, such as Betula ermanii, Quercus mongolica, Alnus hirsuta and Acer mono are seen here and there (Fig. 4). Forty-one species of plants were recorded in 20 plots (3×3 meter square) (Table 2).The dominant species of bird on the island is the Leach's Fork-tailed Petrel (Fig. 2). They begin to come to the island in mid-April and leave at the end of October every year. The petrel digs a tunnel-shaped nest under the ground and lays one white egg. They use dried herbaceous leaves and stems for nesting materials. Takamatsu (1935) observed in 1933 that the first egg was laid July 9. Chicks were found Aug. 19 and then about 80 days after hatching they had fledged.In this survey only eggs were found in the observed nests.Aug. 11, 1956 Abé and Matsuki observed about two-week old nestlings with blackish-grey feathers in the nests.Petrels' nests can be found everywhere on the islet except woods, cliffs and some kinds of vegetation (Fig. 6-a, b). Random sampling was carried out to check the number of nests in each plant community. The numbers of nests in each plant community (3×3 meter square) were 0 to 47 : 47 nests in Artemisia montana community, 33 in Urtica platyphylla, 17 in Polygonum sachalinense, 16 in Cacalina hastata var. orientalis, 5 in Petasites japonicus var, giganteus and no nest was found in Calamagrostis langsdorffii, Ligularia hodgsonii, Miscanthus sinensis community and woodlands. It was found that the difference of the density depended on the vegetation covering on the ground. Some plants, such as Artemisia montana and Urtica platyphylla have very simple root systems, on the other hand, Sasa nipponica, Calamagrostis langsdorffii etc. have very complex root systems (Fig. 8). It is considered that the petrel is not able to dig a tunnel through the complex root systems.Toothpicks were placed at the entrance of the nest hole to check whether the nest was occupied or not on that time (Fig. 9). Four hundred and thirty-seven toothpicks were used for that purpose. Out of 437 toothpicks, 173 toothpicks were missing, i. e. 264 were found in and around the nests. Out of found 264 toothpicks, 97 toothpicks 37% were left as they were, and the remainders (63% 167 toothpicks) were knocked down. It became clear that about 37% of the nests were not used for breeding.The total number of nests on the islet were estimated from the total areas of each vegetation and the numbers of nests on each sampling plot. It amounts to about 1.70×106 nests on the island (370 of them, 1.70×106 nests were not used for breeding).The petrel could be seldom seen on and around the island during daytime, because they are nocturnal (Fig. 7). Diurnal rhythms of the petrel were observed all night long. Numbers of the petrels were counted every five minutes of each hour using two beams of head-lights.
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