Q: We have a medium sized yard enclosed by a four foot fence--will my chickens be able to fly over the fence?
A: Most heavy, docile backyard breeds don't fly very well and aren't particularly inclined to leave a secure, fenced area. A regular picket fence will do to keep many adult large fowl chickens enclosed.
But it won't work for some breeds--some are fliers and will jump fences like they're not even there. Small or juvenile birds will slip between pickets or wires. And a regular 4 foot fence won't necessarily stop predators, either.
So, if there are large dogs or a busy street on the other side of your fence--or if you keep small breeds or breeds prone to flying--you may want to add height to your fence or to clip wing feathers as an additional precaution. You may want to add extra protection along the bottom so no one squeezes through. Some breeds may easily fly low fences and range far. Even if your flock is not in a dangerous area per se, it can be troublesome to neighbors having chickens appear in their yard on on their deck railings--or it may be troublesome to you when they discover your tasty fenced vegetable garden!
Typically, docile, large breed "dual purpose" hens cannot fly as well as bantams or smaller, high strung chickens like Leghorns or Hamburgs, for example. However, young pullets, even when they'll eventually be large breeds, do go through a stage when they have reached their full wingspan (but not their full weight) when they fly higher and more often than they will later when they are older. Most breeds, even heavy breeds, CAN fly a four foot fence if they are motivated. And if they are being chased by something, for example, ALL breeds are far more likely to try to flee their enclosure!
Flee, chicken, flee! Wait, don't.
If their fenced range area is large, your hens may be less inclined to "escape" if there is plenty of room for the number of birds in of your flock. If their yard or run is very small, even breeds that normally tolerate confinement well may be motivated to find their way to the greener grass on the other side. This is especially likely if they have no or little access to good grass or pasture inside the fence, or even if there is plenty of grass, but there are too many roosters and your flock's environment is stressful. They want peace and lots of green, green grass.
In some cases, hobbyists will clip their chickens' wing feathers to help prevent their leaving the yard. (Read the related questions below to see how to properly clip wing feathers.)
Unfortunately, there is no "pat" answer as to whether your chickens will fly a fence or not, just as there is no "pat" answer that will tell you whether or not a dog will be an escape artist, always trying to jump his fence or dig his way into your neighbors' yard. You will simply want to evaluate how much enclosure your hens need given the nature of your breeds, your pasture, your flock dynamic and so forth. You can reduce the likelihood of wayward hens by getting docile, heavy breeds, and by providing them with lots of space and a stress-free environment.