Chickens like to perch. With your top rail solid they have a great place to perch. What you are doing will raise the fence 1'. Raising it is not what will make that work, they could still fly up that high. Having the top of your fence wire instead of a good place to perch is what will make...
You don't manage a tractor the same way you do a fixed coop and run. You don't need as much room in one but you do need to move it often. How often depends on its size, your number of chickens, and your weather. 15 chickens are a lot for this.
I have no idea where you are located so I don't...