The process of asynchronous hatching in the Little Egret Egretta garzetta

1. In order to clarify the process of asynchronous hatching in the Little Egret Egretta garzetta, observations in a large heronry were made from 13 April to 19 July, 1977 in Mie Prefecture. Of 47 observed nests, 12 were investigated in detail by all-day observations for a total of 48 days.2. The breeding cycle was divided into five breeding stages; stage 1 (nesting period), stage 2 (laying period), stage 3 (incubating period), stage 4 (nestling period inside nest) and stage 5 (nestling period outside nest).3. Time interval of egg laying was a mean of 37.13 hours (about 1.5 days), the mean clutch size being 4.86 (range, 4-7).4. Incubation began with the laying of the first egg and was shared equally by both sexes. Incubation rate (the percentage of the incubating hours in all observation hours) increased abruptly, and reached more than 80% (full incubation) after the laying of the second or third egg. Incubation period (days between laying and hatching of the last egg) was a mean of 23.44 days.5. Time interval of hatching was prolonged with the order of egg-laying and was close to the mean interval of egg-laying (about 1.5 days) in the later eggs. The short interval of hatching between the 1st and the 2nd eggs was caused by the delayed hatching of the former egg. This delay will be interpreted as the delayed development of the embryo due to incomplete incubation.