チュウダイサギ

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's one (19.9%), seems to be caused mainly by starvation sequent to the death of the parents.