I didn't think I would have a problem with my six black Australorps because while they were in the brooder with no cover, they never once flew out like I read other people say their chicks did. They would fly to the top of the feeder and waterer, but not out of the brooder box. When they were six weeks old, I moved them to the coop and 25' x 25' run with a 5 ft. tall fence.
I still never had a problem until I started to let them out to free-range part of the day in my woods about a month ago. I live on a little over two acres, over half mature woods. My lot is very long and not so wide, surrounded on three sides by farm land (fenced). On the fourth side is another two-acre lot with a house, which I can't see because of my woods (no fence). The first few times I let the chickens out, they headed straight for the woods until time to come home to roost for the night. I was keeping them penned up until afternoon at first, and that is when they started to fly out in the early morning. They didn't want to wait until the afternoon and discovered their wings.
I thought that was still okay, until the neighbor came to complain because the chickens came in her yard and one came on her deck and she was irate. I just bought this property less than a year ago, and this was the first time I met the neighbor. I had checked on the chickens several times during the day and always found them in a little grove of trees at the edge of my yard, never once thinking they were going to the neighbors yard.
I spent several hours putting netting over the run by myself so they couldn't fly out, and that solved the immediate problem, though the chickens don't like it much. They storm me when I open the gate, trying to get out. They pace in front of the gate waiting to get out. I have always intended to free range them during the day and thought my two acres would be enough for them.
I have since spent over $500 for electric poultry netting and solar charger, but am wondering if they will just fly over that and go annoy my neighbor. If so, I have just wasted $500. I certainly do wish they had never discovered flying.