Influence of Environmental and Density-Dependent Factors on Reproduction of Little Egrets

We evaluated the influence of environmental and density-dependent factors (intraspecific and interspecific) on clutch size, brood size, and nesting success of Little Egrets (Egretta garzetta) in the Camargue of southern France. We recorded these reproductive parameters in most years from 1970 to 1998. We used a generalized linear modeling approach (model selection based on AIC) to examine the environmental effects of spring rainfall, winter temperature, and wind on these parameters. We also examined density dependence of these parameters based on the total number of Little Egrets and the total number of tree-nesting herons nesting in these mixed-species colonies. Clutch size was positively associated with rainfall and negatively associated with the number of Little Egret nests in the Camargue. Brood size was negatively associated with the number of Little Egret nests, although rainfall was only significant as an interaction effect with these two effects. Nesting success was negatively associated with the number of tree-nesting herons, the proportion of each colony consisting of Cattle Egrets (Bubulcus ibis), wind speed, and several interactions among these variables. Virtually all of the reproductive parameters that we evaluated were negatively associated with the number of Little Egret nests or the number of tree-nesting herons. Anecdotal evidence suggests that Cattle Egrets displace Little Egrets at some centrally located nest sites. Such sites are better protected from strong winds, which are a common cause of nesting failure.