Nest building behavior of the Eastern Great White Egret, Egretta alba modesta, was investigated at a suburb of Cheonan City, Chungcheongnamdo, Korea, from 27 March to 31 July 1981 and 3 April 1984. 1. All the egrets nested on the Japanese larch, Larix leptolepis, at 4.6 to 8.7 m (6.1 m on an average) above the ground. 2. One to four nests were built (1.9 on the average) per tree; nest materials were brought chiefly by males (94.4%), while the nest building was carried out almost entirely by females. 3. One nest consisted of 156-282 sticks (212 ones on an average) of such tree species as the Japanese larch (47.4%), the locust, Robinia pseudo-acacia (35.8%), the Japanese red pine, Pinus densiflora (8.5%), the Japanese alder, Alnus japonica (5.7%) and the red oak, Quercus acutissima (2.6%). 4. The Japanese larch showed a much larger annual increment in height than in diameter at breast height (DBH), thus it tended to lose its stability against a strong wind; as a result, most of the nests and eggs fell from the trees. From this fact, the Japanese larch does not seem suitable for the egrets as nesting trees, and this might be the greatest reason for their abandonment of the breeding site. 5. As a protection procedure for the egret, first of all, it is necessary to plant both the Japanese red pine with a low growth rate, but having a thick trunk, stability against a strong wind and power of resistance to the damage caused by their droppings, and the locust with a high growth rate as a stopgap for about 40 years until the planted red pines become suitable for the egret's nesting, in and around the breeding site. And then a strict enforcement of the drastic prohibition of hunting, stealing their egg and invasions into the breeding site by people is demanded. Also, the application of BHC and DDT in Southeast Asia should be restrained.
本報では,韓国忠清南道天安市に渡来・繁殖するチュウダイサギEgretta alba modestaの造巣習性について調査した.1.本種の営巣樹種はすべてカラマツで,巣は地面から4.6~8.7m(平均6.1m)の高さに構築された.2.巣は1本の樹木当たり1~4個(平均1.9個),雌雄の分業によって造られ,巣材の収集は主に雄鳥(94.4%),構築は主に雌鳥によって行なわれた.3.巣は1個当たり156~282本(平均212本)の枝によって造られており,巣材として用いられた枝の樹種はカラマツ(47.4%),ニセアカシア(35.8%),アカマツ(8.5%),ハンノキ(5.7%)及びクヌギ(2.6%)の順に多かった.4.営巣樹種であるカラマツでは,樹高の成長率は胸高直径のそれに比して非常に高く,営巣木はその成長に伴って強風に対する安定性を欠くようになり,そのために多くの巣や卵が落下した.従って,カラマツは営巣に適した樹種とはいえず,これが本繁殖地の放棄をもたらした最大の原因であろう.5.本種の保護・管理対策として,1)成長は遅いが安定性に富み落巣や落卵を起こしにくく,かつサギ類の排泄物による枯損への抵抗性が強いアカマツと,アカマツが営巣に適した樹高をもつに至るまでの代替樹としての,成長の速いニセアカシアとを繁殖地及びその付近に補植・造林すること,2)禁猟,盗卵の禁止及び繁殖地への侵入禁止を徹底的に実施すること,3)東南アジア諸国におけるBHCとDDTの使用を抑制することの3点を提案する.