The first breeding record of the Bridled Tern Sterna anaethetus in Japan

Abstract

The range of the Bridled Tern Sterna anaethetus is widespread throughout in the tropic and subtropic regions of the Pacific, Indian and Atlantic Oceans and, however, the species was regarded as a rare straggler in Japan.We discovered the breeding of the Bridled Tern on Nakanokami-shima, Sakishima Archipelago southernmost district of Japan during our stay for ringing sea-birds from 28 to 30 June 1980. Nakanokamishima, another common vernacular name Uganjima, is situated at 24°11'N, 123°34'E, 15km SW of Iriomote-jima and small (length c. 1800m, width c. 300m, alt. 102m) uninhabited island. Here is one of famous sea-birds islands of Japan, yet the conservation is so poor that the sea-birds colonies are often destroyed by Formosan who steal the eggs for food. The breedings of Calonectris leucomelas, Sula leucogaster, Sula sula, Sterna fuscata and Anous stolidus have been known here.We observed the Bridled Terns at three areas, A, B and C, of the island (Fig. 1) and it seemed that all of them were breeding. The areas A and B were near the shore-line, and large rocks fallen from steep slope heaped up all over there. We could count about 60 pairs of the Bridled Terns at the area A (c. 30 × 100msq.), and only the species were breeding in the colony. In the area B, about half of the area A, the Bridled and Sooty Terns were settled, and a number of the Bridled Terns were about 20 pairs. The area C was rocky place near west end of narrow ridge of the island, and several pairs were breeding.We searched the breeding status of the Bridled Tern a little intensively in the colony of area A. All ten clutches found there contained one egg, like in other foreign colonies. Their eggs were laid on either inner floor or open flat surface of the rocks more or less covered with large rocks (Photographs 1 and 2 of Plate 6). As described by Warham (1958), the Bridled Tern colonies on Nakanokami-shima were much smaller than those of the Sooty Tern, and most of the Bridled Terns were incubating while all of the Sooty Terns were taking care of their chicks.Two species of Sterna and one Anous breeding on the island were apparently segregated from their nest-sites. The Sooty Tern was most numerous and some thousands of pairs bred. Most of them made a colony on the plateau of short grass on the center of the island. On the other hand, the Bridled Tern selected such rocky place as mentioned above. The Common Noddy Anous stolidus settled on the ledge.Analizing other data after this discovery, it became known to us that probably the Bridled Tern had bred Nakanokami-shima and Hamano-shima, 5km NW of Taketomi-jima, few years ago, and that the species might be breeding on Hude-Rock, 12km NE of Seto-zaki, Miyako-jima.