Estimation of the breeding population of the Sooty Tern Sterna fuscata on Nakanokami-shima, South Ryukyu, Japan

Abstract

During our surveys on Nakanokami-shima from 1975 to 1984, seven species of sea birds were recorded breeding. Among them, the Sooty Tern Sterna fuscata was probably the most abundant. Fig. 1 shows the locations of colonies of the species in 1981, 1982, and 1983. The photographs were taken from selected high points situated west and east of the main colony. Based on these photographs, the number of the birds which stayed on the colony in the daytime was counted; while the birds on subcolonies were counted through binocular. The results of counting are shown in Table 1.
Attendant birds on early stage of breeding season were adult plumaged. Most of these birds remaining during the daytime seemed to be occupying nest sites, and it seemed that one bird of each pair stayed on the nest site while other bird was absent offshore. If the number of non-breeding birds, including immatures, was negligible, then the number of birds staying on the colonies in the daytime should be equivalent to the number of breeding pairs. On the above presumption, the breeding population of the main colony were c. 6000 pairs in 1981, c. 3500 pairs in 1982, and c. 2500 pairs in 1983. This remarkable annual variation is due to the fact that the centre of the main 1981 colony was covered with tall dead herbage in early 1982; and with well grown grass in 1983 (Fig. 2). The changes in the vegetation resulted in fewer pairs settling in the area. On the other hand, neither the apparent increasing of breeding density as a whole nor the expansion of colony did occure in 1982, and in 1983.
In 1982, the number of fledgling in the main colony on 22 August was c. 1800; approximately 0.51 chicks per pair. However on 9 August, 1982, a strong storm (Typhoon-8211) had passed the island and 300 dead chicks were found in the main colony on 18 August. Therefore, the production of chicks per pair just before the accident was probably at least 0.60.
The estimations of total breeding population in 1981, 1982, and 1983 were 6400 pairs, 4100 pairs, and 3000 pairs respectively.