[A comparative study on asynchronous hatching and nestling growth of three heron species]

The study on the asynchronous hatching and nestling growth of three heron species (Ardeola bacchus, Egretta garzetta, Nycticorax nycticorax) in laboratory and field at the Changshan and Yuhang heron reserve in Zhejiang Province in 1996-1999 showed that the herons migrated to the reserve in the first and second ten days of April, and left in late September. The average hatching period was 23.0 days for Ardeola bacchus, 23.9 days for Egretta garzetta, and 25.3 days for Nycticorax nycticorax. Nycticorax nycticorax had the highest hatching rate (95.45%), followed by Egretta garzetta (86.98%) and Ardeola bacchus (76.23%). The hatch ability of Nycticorax nycticorax (3.50) was higher than those of Egretta garzetta (3.38) and Ardeola bacchus (3.21). The increase of nestling weight was negatively related to the adult body size, but positively related to the hatching order. All nestlings developed well at their early growth stage, and then, there existed a significant difference among the nestlings based on the hatching order. The growth curves of the first three nestlings were similar and obviously better than that of the fourth, which indicated that early-hatched nestlings could obtain enough food, while the last one might be short of food. The optimal hatching quantity of the three heron species was assumed to be three offsprings.