鳥島初寝崎のアホウドリ新営巣地の安定と個体群増加

抄録

From 1991 through 2019 the total population of Short-tailed Albatross Phoebastria albatrus on Torishima increased by 8.5% annually, from c. 500 birds in 1991 to c. 5,000 birds in 2019. In order to decrease extinction risk associated with volcanic eruption, a new breeding site was artificially established at Hatsunezaki using social attraction. One pair bred every year at Hatsunezaki from 1995 through 2003, and four in 2004. Thereafter, the number of pairs increased by 29.1% per year, and 389 pairs laid eggs and 285 chicks fledged in 2018/2019. This high population increase rate was mainly attributable to immigration from the source population at Tsubamezaki on Torishima. Observation of 791 color-banded fledglings from Hatsunezaki showed that they first visited this site at 3.4 years-old, mated for the first time at 4.6 years-old, and bred for first time at 6.2 years-old, with no sex difference. Mean annual death rate was estimated to be 6.6% per year. Using the information of the resident time of individuals, the number of landings taking place every hour and at sunset, the frequency and position of courtship behaviour, and the maximum number of landings, it was possible to predict the number of pairs present several years later.