Let's be honest: if you have ever owned a chicken, you already know they answer to a higher power, and that power is chaos. The moment you think you've outsmarted them, they squeeze through a gap you swore was air-tight and waddle into the one place you told them not to go. This is the story of one such hen, and honestly, it is the most relatable thing we have read all week.
The OP has a careful, loving setup: chicks in one pen, adults free-roaming and friendly, a nightly routine that keeps everything running on autopilot. Then one adult hen decides she is made of liquid and pops through a gate gap that should, by all logic, be too small for a bird of her particular girth. She trashes the chick pen, scattering dirt, dunking feet in the waterer, leaving divots like a tiny feathered bulldozer. And now there's a real problem: one chick has coccidiosis, and the last thing anyone needs is the whole flock getting sick because a hen thinks she's a contortionist.
So what do we say to someone who asks us for advice on this? First, stop thinking of that gap as a gap. It is not a mistake in your fence. It is a chicken-sized open door, and your hen has already memorized the exact angle required to slip through. Adult chickens are surprisingly collapsible, they can compress their keel bone and fluff down to nothing when they really want to reach forbidden territory. You need to close that gap with something solid: a scrap of hardware cloth, a zip-tied piece of plywood, even a brick wedged in. Do not trust your eyes. Trust her ambition.
Second, the coccidiosis risk is real and you are right to be worried. Adult chickens can carry the protozoa without showing symptoms, and if they keep waltzing into the chick pen, they will track it right in. So until treatment is done and the chicks are older, think of that gap as a biohazard seal. Block it. Tape cardboard over it if you have to. Your annoyance is fair, but your instinct to protect the chicks is the real priority here, and you have the power to fix this.
Here is the concrete truth: that hen is not being malicious. She is being a chicken. They are creatures of stubborn habit, and once they decide a spot is theirs, you have to physically remove the option. Do not outsmart her. Out-block her. One afternoon with a drill and some wire mesh, and your "chick-proof plan" stops being a joke and starts being a solution. Then you can go back to loving your feathered friends from a safe, gap-free distance.