Pekins can't fly. They can glide but not take flight. I used to raise them and they are so friendly and sweet. I would let them out during the day when I was home and they would come up single file to my front door quacking for a treat, then they would head back down toward the pen in single file. Loved it! Critters will definitely get them at night without a top on the pen, hence the term "sitting duck". I would cover it with something.Okay a pen for fat and happy pekin ducks - does it have to have a top to it?