Chickens do have vitamin, mineral, and macronutrient requirements to maintain health, produce eggs, and produce the next generation of healthy chicks. So it's not just "anything" in that sense. Some areas couldn't support sustained, particularly large-scale, flocks with only inputs from the local environment. That said, they're not too finicky.

Secondarily, whole flocks, commerical and backyard, are being culled to attempt to contain avian influenza. One human recently died after getting it (they had a backyard flock and also other health problems).