How far are you willing to go?
If you can put down some kind of barrier that will allow drainage into a sump, you could literally put down soil and seed it all in hardy, shade-tolerant edible groundcovers.
If you added grow
lights to the space, or if the light through the windows was
enough, you could get more ambitious.
I would grab some large rubber trays, like trays for sodden
boots, drill some drainage holes, and try some trays with different seed mixes that would work for your birds. You might find yourself using
chicken wire or something to isolate the
root zones from scratching and pecking, such that the "pasture trays" can grow back, but you could also overseed at need, considering the space involved.
Let us know what you decide, and good luck.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein