So just from my own experience, someone will probably fly out. IF one flies out it may panic and struggle to remember how to get in, so when using a wire surround like this, I find it safer to elevate it slightly, and let the chicks come and go from there as they wish.
So hard to tell from this photo (because I snapped it without actually setting it up), but the bricks around the perimeter of my brooder are there to rest the chicken wire on top of, and create openings all the way around, so the chicks can duck in and out but adults can't get in.
(Forgot to mention, chicks here are 15 days old).
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