Duck Yard to Chicken Yard - Help!

A few weeks ago, I moved my dozen chickens (Wyandottes) from the tractor pen (which they have outgrown) to share the duck yard. This is a 40x40 yard surrounded by a 4 foot chain link fence cemented into the ground. Never had a duck escape or a predator problem. They have a nice coop (8x8x8 with roosts inside) all to themselves (the ducks have a separate shed) - which they are shut into at night. BUT, the chickens have figured out how to escape the yard -- they jump to the top of the fence, then down the other side! Saturday the dog got one, so now they are all confined to the coop until I can figure out how to keep them inside the fence. What can I add to the top of the fence to keep them in? How much higher would it need to be? They aren't actually flying over (tried clipping wings the first time they escaped) they are 'jumping' to the top of the fence and then down. I need something higher and that will prevent them from landing on the top. Thinking about adding sticks and twine topper adding about 2 foot in height and leaning into the pen at a bit of an angle. Or maybe a chicken wire topper (more expensive, but would look better) adding similar height but left loose enough that it will give under their weight and dump them back into the pen. Anyone with similar experiences who can tell me what pitfalls to avoid?