building a coop and run question

[cmacks]

I just recently got 6 chicks and 5 turkeys. I would like to get some ducks as well. I have a 100x100 ft dog pen that my three dogs only use a portion of. I was thinking of sectioning off 30x30 and making 3 different runs for each breed. I own 3 acres and have a bayou on the side of my house with a couple hundred acres on the other side. I have alot of racoons and hawks. I was thinking of making the 30x30 pen an island in the middle of the dog pen so the dogs would keep out predators. My question was whether or not to put a top on the pen. I wouldn't mind the ducks being able to fly to the bayou but worry about them coming back to the coop. Any suggestions? Thanks Craig