Minecraft

Can baby chickens fit through half block gaps? Do you have any one block gaps in that structure? Hard to tell with connected textures on and that all being glass.

Long long ago I got killed because I didn't know zombie children were a thing and one got through a one block gap.

Maybe they glitch throug the corners when growing up. Try making the glass skirt two blocks high and fill in the bottom right corner.

Are you breeding them, or throwing eggs in through the top, or have some automatic egg throwing mechanism? (I don't see the latter but it looks modded so I'm not sure what could be there). If it's throwing eggs related, I think from time to time I've had baby chickens spawned that way end up inside the side if a block where they immediately suffocate. I'm not sure glass has that suffocating effect because it's a transparent block, so maybe some of the chicks spawn glitchily partly inside the glass and some of them manage to get out the other side of it somehow?

Reminds me if the time when our kids were playing on the family server. I had built a highly automated but also fancy chicken farm. I had forgotten to "turn it off" before logging out, so the chicken multiplied fruitful while the kids were playing, dragging down the server.

My daughter could have turned it off, eliminating all free chicken, etc. by pulling a appropiately labeled lever. But no, to kill the chickens, she poured a bucket of lava into the court.

Yes, it was placed correctly to kill all chicken. But it also burned down the complete construction, which was made of wood...