干旱区生态环境知识资源中心

Within the Murray-Darling Basin of eastern Australia there has been a general decline in the abundance of wading birds. Loss of wetlands caused by river regulation and irrigated agriculture is considered to be the main cause. We assessed the adequacy of irrigated rice fields as substitutes for natural wetlands for foraging egrets during the breeding season in one study area around a mixed colony of egrets in southeast Australia. For aerially sown rice crops, which represented 85% of all rice in the general area, the densities of foraging Intermediate Egret (Egretta intermedia) and Great Egret (Ardea alba) tended to reach a maximum four to six weeks after sowing of the rice crop, and declined thereafter. The decline phase was correlated with decreasing prey capture rates and a shift from vertebrate prey to less profitable invertebrate prey, and coincided with the start of the egrets’ main chick rearing period when food demands would have been high. Even at maximum densities it was estimated that only 5-13% of the Intermediate Egrets and Great Egrets that were available to feed actually did so in the rice growing areas within 6.5 km of the colony. In contrast, Cattle Egret (Bubulcus ibis) densities in rice fields did not decline until after their young fledged. They fed almost exclusively on insects and their prey capture rates showed no seasonal decrease. Up to 60% of the available Cattle Egrets were found feeding in the rice fields within 6.5 km of the colony. It is concluded that rice fields provide a good foraging habitat for the Cattle Egret and may be contributing to the spread of this invasive species, but that they are probably not a preferred foraging habitat for the Intermediate Egret and the Great Egret.