How high CAN they fly and how high WILL they fly are two separate issues. Heavier breeds, once grown (because young pullets are lighter and fly better), can generally be contained within a 4 ft. fence. Lighter breeds can go quite a bit higher. Many say that with wire fence with no visible top or ledge appears to be much higher to a chicken, because they can't really judge how tall it actually is - there's no "goal" to fly to (or over). All of our fence is 4 ft, and none of my chicken have ever flown over it. Now, my EE flew on top of our (covered) run with no problem, and it's 6 ft. high. But she was trying to get away from a harrassing rooster. She never flew over our 4 ft. fence. Basically, as long as there is plenty on their side of the fence to keep them happy, they usually will stay there. You can also clip wing feathers to help, if they end thinking the grass is greener on the other side....