They keep him drugged up, and he's still barely controlled. Without the darts every 3 hours, he'd tear everyone else to shreds just to feed the hunger within him.
Makes sense. Evolution gives prey animals eyes with as wide a field of vision as possible, so they can detect predators better. Elephants are too large for predators to mess with and so is big bird.
I had to go look it up. Not sure that picture is accurate, mind, it’s the only one I could find. (Though lots of comments about herds being led by a blind elephant…. I’m sure there’s a joke in there.)
We know from Big Bird's extended family that his species has a wide spread of individual variation, and given that none of them reside in the wild none of them are likely to suffer predation due to what would otherwise be a mal-adaptation, providing a springboard for even more genetic variation from generation to generation.
Just look at how wildly different looking specific humans can be from one another, even within a single community.