Can Emus, Rheas, and Ostriches thrive on just grasses and water?

[KT9278]

(how would this affect their egg laying - I know it would go down but by how much?)

 

[JacinLarkwell]

(how would this affect their egg laying - I know it would go down but by how much?)

Thrive, very much doubt it. Survive, probably not. I don't think even that's what they solely eat in their natural environment