I have a smallish back yard in the city with 4 ft chain link on one side and 6 ft chain link on the other. My chickens have never crossed the 6 ft side, but they regularly flew over the 4 ft side (Barred Rocks, Easter Eggers, an Australorp, and a RIR), especially when my neighbors put their bird feeder out. I used plastic ties to attach a 2 ft extension (you can barely even see it, but the chickens sure can!), and I haven't had any problems since! Make sure to check for holes in the bottom too, especially when they are little.