エリグロアジサシとベニアジサシのモニタリング手法の提案

—コロニー外からの観察による営巣数の計数と雛の齢査定に基づく産卵時期の推定—

抄録

The Black-naped Tern Sterna sumatrana and Roseate Tern S. dougallii are inshore breeders on the Ryukyu Islands, Japan. Monitoring surveys are an important tool in conservation measures for these species, yet it is essential that disturbance to breeding caused by entering into tern colonies is minimized. Based on our long-term studies of the breeding ecology of these two species, we suggest methods of counting of the nest number and estimating the egg laying period based on estimated age of chicks derived from observations made outside the colony. The minimum number of nests can be assessed by counting the number of adults sitting on nests, and the number of chicks out brooding by parents, or fledglings. The hatching date can be estimated by observation on characteristics of their figure and behaviors. The egg laying date can be back-calculated based on the incubation period, which averages 24 days for the Black-naped Tern and 23 days for the Roseate Tern. Furthermore, it is possible to compare the timing of the start of the egg laying and that period between colonies. It is desirable that more than one researcher divide the breeding sites and conduct the above survey on each breeding sites at the same time.