Nest Numbers of Five Ardeids in Hong Kong, South China, 1989-2004: Does Weather Affect The Trend?

Abstract

This study reports on the number of nests and colonies of Great Egret (Ardea alba), Little Egret (Egretta garzetta), Cattle Egret (Bubulcus ibis), Chinese Pond Heron (Ardeola bacchus) and Black-crowned Night Heron (Nycticorax nycticorax) in Hong Kong between 1989 and 2004. Little Egret, Chinese Pond Heron and Black-crowned Night Heron were the dominant local breeding ardeids. Regression analysis of the number of nests and colonies with year revealed that the number of nests of Great Egret, and the number of colonies of all ardeids, except the Black-crowned Night Heron, showed a significant increase. The number of nests of Great Egret increased from no nests in 1989 to 85 in 2004. The other species fluctuated from 100 to 300 nests each, except for Cattle Egret which stayed steady at about 100 nests. Regression analysis of the relationship between climate variables and residuals, after removing the effect of any long-term trend, shows that there was a significant positive association of the number of nests of Cattle Egret with total rainfall during breeding. The increase in numbers of colonies, but not the number of nests, indicated that previously large colonies had split into smaller ones, presumably due to loss of feeding and nesting habitats. The El Niño events in 1991 and 1997, but not in 1994, coincided with declines in nests. The magnitude of El Niño may be related to the impact on nesting ardeids, but other local confounding variables, in particular feeding habitat availability, should also be considered. The number of Great Egret nests in Hong Kong is of regional importance, as it is about half the known nests in southern China. It is recommended that monitoring of breeding success should be initiated in major colonies, as well as an integrated wetland monitoring for Deep Bay and Starling Inlet.