Breeding Ecology of the Eastern Great White Egret, Egretta alba modesta, in Korea : III. Egg-laying, Incubation, Brooding and Growth of the Chick.

At a suburb of Cheonan City, Chungcheongnamdo in Korea, egg-laying, incubation, brooding and growth of the chick of Egretta alba modesta were investigated during the breeding period from April to August 1981. 1. In 25 nests, the clutch size averaged 3.8 eggs (range=2-5), the interval between laying of successive eggs being 44.5 hours. Egg weight, in which the first egg was heaviest (50.0g, N=25), decreased in the egg-laying order. 2. Incubation, mainly by the female (80.3%, N=5), began immediately after laying of the first egg in all the nests (N=238). The time required for incubation and brooding by the female diminished gradually after hatching, and thus became almost equal to that by the male, this suggests that a need for the female to sit on the nest reduces. The incubation time was longer on rainy or cold days than on warm, sunny days. 3. The incubation period was 23-29 days in the 25 nests, and eggs were hatched in almost the same sequence they were laid. Sixty percent of the eggs fell from the nest, and the hatch-ratio was very low (only 40%) at the breeding site. 4. In the first chicks (N=5), the growth rate of the wing was highest as compared with those of the culmen, tarsus and tail, which is regarded as an adaptation for fight. 5. The adult mortality during the breeding season was higher from April to June (5.8-7.2%) than in July (2.2%), which is probably attributed to a great expenditure of energy due to the inward migration, incubation and feeding, and also to the acute poisoning to death by organochlorine compounds. The high chicken mortality (43.4%), nearly twice the adult\u27s one (19.9%), seems to be caused mainly by starvation sequent to the death of the parents.韓国中部地方の天安市で,1981年4月から8月までの繁殖期におけるチュウダイサギEgretta alba modestaの産卵,抱卵,抱雛及び雛の成長について調査した.1.本種の25巣における一巣卵数は2～5個(平均3.8個),産卵間隔は平均44.5時間であった.卵重量は第1卵で最も重く(平均50.0g,N=25),産卵の順に減少した.2.抱卵は全巣(238巣)において第1卵産卵と同時に始まり,調査した5巣では主に雄親鳥によって行なわれた(80.3%).雌の抱卵･抱雛時間は孵化後には短くなり,雄のそれとの間に殆ど差を示さなくなった.このことは,抱卵を終えた雌の巣に留まる必要性が少なくなったことを示唆している.また,抱卵時間は晴天日よりも雨天や低温の日により長いことが知られた.3.調査した25巣において孵化は産卵後23～29日目に行なわれ,孵化順は産卵順とほぼ一致した.全巣における本種の落卵率(60%)は高く,従って孵化率(40%)は非常に低かった.4.供試5羽の第1雛では嘴峰,跗蹠及び尾の成長に比して,翼の成長速度が最も速く,これは飛翔に対する一つの適応と考えられる.5.繁殖地における調査期間中の成鳥の死亡率は7月(2.2%)に比し,4月から6月(5.8～7.2%)に高く,主な死亡原因として渡り,抱卵及び育雛などによる体力消耗と有機塩素化合物による急性中毒死が挙げられる.また,雛の死亡率(43.4%)は成鳥(19.9%)に比して約2倍も高く,親鳥の死亡による餓死がその最大原因であろう