Darwin's Nothura consumes both plant and animal material (Bump and Bump 1969). Bump and Bump (1969) identified 23 genera and 19 species of plants in the diet in Argentina, including many species of cultivated plants. Garitano-Zavala and Ávila (2003) reported over 50 species of plants in the diet in Bolivia. The plant material in the diet includes both seeds and leaves. The most frequently consumed plants reported in Argentina by Bump and Bump (1969) were species of Asteraceae, Brassicaceae, Chenopodiaceae, Geraniaceae, Papilionaceae, and Poaceae, and in Bolivia reported by Garitano-Zavala and Ávila (2003) were species of Geraniaceae (Erodium cicutarium), Leguminosae (Trifolium), Malvaceae (Tarassa tenella), Poaceae (Agrostis, Festuca, and Paspalum pygmaeum), and Rubiaceae (Relbumium). Garitano-Zavala and Ávila (2003) also noted that "very few items were consumed in high proportion", and that, at least in the nonbreeding season, the diet of Darwin's Nothura overlaps broadly with that of the much larger Ornate Tinamou (Nothoprocta ornata).
The most frequently eaten insects are ants (Hymenoptera), beetles (Coleoptera, including Curculionidae and Carabidae), grasshoppers (Orthoptera), Hemiptera, Homotera, Lepidoptera (caterpillars), and Diptera (both adults and juveniles) (Bump and Bump 1969, Garitano-Zavala and Ávila 2003). Garitano-Zavala and Ávila (2003) also that mouse feces commonly were encountered in crop contents in Bolivia.
Open water for drinking is scarce in many places where Darwin's Nothura occurs, at least in Argentina; in captivity, nothuras frequently obtained water from dew (Bump and Bump 1969).