Good morning!
@Duckworth has given you some great ideas!
I know you're wondering why you have to worry about frostbite in the pen. When ducks breath the let off a ton of moisture so even in the dead of winter when it's below 0° if you don't have really good ventilation it will cause moisture to build up in their pen to the point that they get their little legs and feet frostbitten.
When I saw the picture and the big barn I got excited!

I do think I see a piece of that white vinyl fence that will probably go to your property line? It looks to me that you've got plenty of room for 3 ducks in that area so I wouldn't worry about that. The people I got my first 4 ducks from had a ton of ducks in a space much smaller than what you've got. Maybe you could attach your duck pen to that existing shed and use the side of it as one of the walls of your pen? Since your man is in construction I'll sure he'll figure it out. 2 sides of both of my pens were sides of the barn which made it simple to construct. My husband was talking about building a 3rd pen between my 2 exisiting ones which would basically just mean putting a floor and top on it because he'd use the sides of the 2 pens I've already got.
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