Rheidae eggshells are abundant in the archaeological record of west central Argentina. Eggs were more important in human diet than the bird meat itself, as with many other Ratite species around the world. Currently, two species inhabit the south and central areas of Argentina and Chile (Rhea pennata and Rhea americana), with different geographical distributions and diet but with similar behaviour. In this paper, we use δ13C and δ18O isotopes from Rheidae eggshells to explore the ratite diet, its spatial differences in the archaeological record, and their importance in human diet, and as a way to approach human mobility in hunter‐gatherer societies from west central Argentina. There was a significant correlation between both δ13C and δ18O isotope data and the Rheidae species' distribution in the landscape, showing differences in their past diet and territoriality. Additionally, the unexpected δ13C and δ18O isotope values in the Altoandina Desert area suggest human transport of the Rheidae eggs to these locations. The isotopic data from the eggshells need to be incorporated into the ecological history of the region in order to understand past human diet and mobility. Copyright © 2013 John Wiley & Sons, Ltd.