In the Camargue of southern France, declining nest success of the Little Egret (Egretta garzetta) in mixed colonies over the past three decades has been suggested to be, at least partially, explained by exclusion from high-quality nest sites by the Cattle Egret (Bubulcus ibis). The Cattle Egret has been rapidly increasing over the same time period and are well known to be aggressive at the nest. Previous studies have also shown that the success of the Little Egret is highest in the centers of colonies. The behavioral interactions and spatial patterns of nest establishment were studied within colonies of the two species. Our data suggest that the Cattle Egret was more successful at displacing the Little Egret at nest sites than the reverse. Three indicators of nest-site quality (i.e., laying date, nest density and position within the colony) were also used to test whether the proportion of Little Egret nests within different sections of a given colony was less than that of Cattle Egret nests at higher quality sites. Our results indicated that the proportion of Little Egret nests at a given site decreased with increasing site quality. Thus, the data are consistent with the hypothesis that the Cattle Egret may exclude the Little Egret from using high-quality nest sites at least in some colonies during some years.
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Vol. 29 • No. 2