Asynchronous Hatching, Growth and Survival of Chicks of the Cattle Egret Bubulcus ibis

Abstract

(1) Asynchronous hatching, growth and survival of chicks in the Cattle Egret Bubulcus ibis were studied at a heronry in central Japan in 1978 and 1982. (2) The average clutch size was 4.34 in 35 marked nests (studied only in 1978). Eggs were laid in the early morning at an interval of two days (70.3%) or more, averaging 2.27 days. (3) Incubation behavior by both sexes started gradually a few days before first egg-laying. The average hatching interval was 1.88 days in 1978 and 1.93 in 1982. (4) Both laying and hatching intervals tended to be prolonged in the latter half of the laying and hatching periods, respectively. (5) The average hatching spread (data combined from the two years) was 6.3 days (N=72) with a maximum of 16 days (three instances). (6) The first and intermediate chicks grew equally irrespective of their own brood size. Growth rates of the youngest chicks in B/4 and B/5 in 1978, as well as in B/3 and B/4 in 1982, were lower than older siblings. (7) More deaths were recorded amongst d- and e-chicks in B/4 and B/5 than amongst a-, b- and c-chicks. It was likely that the former died of starvation or siblicide, whereas the latter died of various accidents.