Grasses and insects provide the staples of the Greater Rhea's diet. Their huge caecum and colon may act as fermentation chambers producing volatile fatty acids, digesting hemicellulose and cellulose with the help of bacteria (Martella et al. 1996; Sales 2006).
In agricultural areas Greater Rheas eat mostly alfalfa and wild dicots, with no preference for grasses. Rheas consume higher proportions of alfalfa in the winter, when the resource is too short to be used by cattle, and thus pose no competition for cattle grazing opportunities (Martella et al. 1996).
Early reports of rhea diets also mention the consumption of berries (Stenjneger 1885), and snakes and rodents (Peterson 1963: 179). Rheas twice have been reported pecking at fresh fecal matter dropped by other rheas. One such bird proceeded to walk away and wipe its bill on grass, a movement common in passerines but not previously reported in rheas (Raikow 1968).
Raikow (1968) also reports two occasions in which rheas tried to catch small birds, including a vividly described scene in which a captive rhea succeeded in killing and eating a Common Grackle (Quiscalus quiscula) in the Detroit Zoological Park. After an excited period of running around its enclosure during which it stopped often to rub its prey in the dust even after the grackle had stopped moving, the successful predator ran off with its prey after attacking another rhea who attempted to steal it. After 15 excited minutes of extreme vigilance, it ate pieces of meat torn off the grackle by pecking and shaking it in the air. With vigorous head-shaking the rhea was able to swallow the remnants whole (Raikow 1968).
Dietary breakdown of wild Greater rheas
(from Martella et al. 1996):
Green material: 90.1%
Insect fragments: 0.1%
Feeding captive chicks either 0.7 or 1.4% dietary calcium has been shown to improve growth rates without causing bow leg syndrome (Bastos et al. 1999).
Morata et al. (2006) analyzed the metabolizable energy values and digestibility of dry and organic matter in several feed ingredients produced in Brazil, using the total collection of feed intake and excreta output or external marker chromium oxide on 7 month old Greater Rheas. This information may prove useful in determining the optimal cost-effective feed formulation for captive birds.