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The food and foraging strategies of the Lesser Rhea (Pterocnemia pennata) were studied in the high arid pampas of the Andean Precordillera in Mendoza, Argentina. The Lesser Rhea is primarily herbivorous and the composition of, and seasonal changes in, the diet were analysed in relation to the availability of vegetation. The analysis of vegetation and sampling of faeces were carried out in spring, summer and autumn 2002-03. Grasses and shrubs dominated the vegetation of the study area, while forbs and cactus (Cactaceae) were present only in low proportions. The main dietary component was leaf matter (94.30%), with the rest seeds (5.66%) and a small proportion of insects (0.03%). Stones constituted 2.24% of the dry weight of faeces. The diet included 70% of the available plant species in the area. Rheas showed a preference for forbs and shrubs in relation to the availability of these plant-types and, while grasses were the dominant plant-type in the environment, they were not selected in relation to their availability; Rheas showed no preference for cactuses. Dietary diversity (Shannon-Wiener index of diversity of plant species in the diet=0.77) exceeded environmental diversity (Shannon-Wiener index=0.29). The diet of the Lesser Rhea in the Andean Precordillera of Mendoza was almost completely herbivorous and generalist, although selective for some plant species, features that would constitute an adaptation to survive in this clearly arid environment.