oh man, anything. If they're spoiled they get snooty, but when I forget to provide fresh greenstuff for my birds for a little, they'll eat just about anything as long as it is not tenuous and woody like tree trimmings. I threw in some artichoke palms (the big leaves of the plant) the other day and they were devoured. Path of least resistance? Provide water and sunshine and let whatever weeds that are there already grow fresh and green. Chickens love oxalis and any sort of grass which is what grows rampantly in my yard.
If you really want to put the work into planting something, collards, kale, mustard, bok choy, all of the above. Maybe what would be best is fast growing vegetable with an easily germinating seed that you can direct seed into your veggie beds rather than transplant, for instance radish, or salad mix.