My father had chickens when I was a kid, and one of the major drivers for him getting them was that he raised a LOT of sweet corn in his large garden.
Corn likes a high nitrogen fertilizer, and chickens are REALLY good at making garden scraps (including the little bits of corn left on the corn cobs from all the corn we ate for dinner) into high nitrogen fertilizer.
Obviously, the chickens got other food too (garden season is far too short up here in the northeast), but the chickens feeding the garden and the garden feeding the chickens always struck me as doing "sustainability" the right way!