I have a big fat cochin cross... I watched all 10.5 lbs of him clear a 5 foot tall fence... from standing...
I do believe that they will spend more time pacing back and fourth by a fence before they will jump out and over it when compared to a wood fence. That said, I have temp fencing up to keep birds on ground flattening duty, and some of the fence is 18 inches tall... and is enough to keep them mostly in...matter of fact, when they want out, they jump up 4-5 feet to the wood fence that is on one side of the fenced off area.
Keep them entertained inside the fence, and they will be less likely to leave. The fact that my adult birds are on the outside of the fence, probably impacts why they feel safer in the fence!