I have 5 muscovy ducks, two 6 weeks and three 4 weeks old. They have a coop that they obviously get locked in at night and the coop is penned in but does not have a cover on top. WE live in the country and are surrounded by bush. I'm not sure if its a good idea to leave the coop door open and let them go in and out as they please. I get worried about predators like hawks but if I'm with them and they even see an airplane they will go back in the coop. It gets quite warm in their coop so I've been going down about 3 times (sometimes more) a day to let them swim in their kiddie pool but I really would rather just leave their door open to the penned area instead of worrying all the time.
Without a cover of some kind over their penned area, they could be vulnerable to hawks, owls, cats, dogs etc..... BUT many of us do let our flock out in an uncovered run, you just have to know your predators. My penned yard is uncovered, but has almost total tree canopy, so I don't worry about hawks. I do keep my eye out for cats and the ocassional loose neighbor dog. You just have to decide what you're comfortable with.
Being in the country, you might want to add additional birds that help watch out for predators. I read that geese and guinea fowls are good for this.