You'd have to look up how to do it, but you could clip their wing feathers. That keeps them from being able to fly.
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I've learned from personal experience that it's better to save up and put in the money to build the best pen or coop you can from the start. I have a few really huge, fat hens in my flock, and they could all fly over a 3-4 foot fence even with their wings clipped. I've seen my Leghorn-mix hen fly over my garden--6 feet high for over 40 feet across the yard--WITH her wing clipped according to all the instructions I've researched. I agree that generally, if your chickens are content in their pen, they will not try to fly out. But..If a chicken gets very excited about something beyond the fence, or gets badly spooked by something, she will find a way to fly to places you would never think possible. In the last 3 years, I have not had any chickens escape over the 5 foot fence in our yard, but they have over an acre to run on with many big bushes and hiding places. I think my Cochin is the only hen I have that could possibly be contained by a 4 foot fence. Good luck and take good care of your chickens!I think I'm going with a 3 Ft fence, cause that's all I can afford right now. But I might get a 4-5 Ft fence this summer.