Trophic conservatism in breeding Western Cattle Egrets (Ardea ibis) across a Mediterranean climatic gradient

Abstract

Understanding how environmental gradients shape the foraging ecology of generalist waterbirds is essential for predicting their responses to climate and land-use change. We investigated the breeding-season diet of the Western Cattle Egret (Ardea ibis) across semi-arid and sub-humid Mediterranean regions of northeastern Algeria using pellet analysis from nine breeding colonies. Diet composition, prey diversity, prey abundance, and trophic overlap were quantified using colony-level statistical analyses to account for the hierarchical sampling structure. The diet was overwhelmingly dominated by invertebrates, particularly Coleoptera, with Orthoptera representing the second most important prey group, whereas vertebrate remains occurred only occasionally. Breeding colonies located in sub-humid regions exhibited greater prey diversity and a higher occurrence of Orthoptera than colonies in semi-arid regions. In contrast, differences in overall prey abundance and vertebrate occurrence were not supported after accounting for colony-level variation. Despite these climatic differences, dietary overlap among colonies remained consistently high, indicating that breeding Western Cattle Egrets maintained a highly conserved trophic niche across contrasting environmental conditions. These findings demonstrate that climatic variation influences the diversity and relative use of prey rather than fundamentally altering the species' feeding strategy. The ability to adjust the composition of the diet while maintaining a stable trophic niche likely contributes to the ecological success of the Cattle Egret in Mediterranean agroecosystems. At the same time, continued monitoring of colony-level dietary variation will be important for understanding how ongoing climatic and land-use changes may affect prey availability and trophic interactions in this widespread colonial waterbird.