Ducks in -40C, duck mum freaking out

[Miss Lydia]

But if you can provide shelter that is much better.

 

[Lilbitsnpieces]

So its the middle of our great Canadian Winter now and its currently sitting at -38C WITHOUT the windchill. Feels like -44C.
I have 8 ducks in my 8x12 coop. Lots of thick straw, heated water buckets and a little shrub cover to offer some smaller area for them but man I am freaking out. They are all huddled in the corner, some have poop frozen to their butt feathers, but they're still happy to eat and drink. They keep plopping their feet back into their feathers though.

Do I bring them inside for the night? I dont have a heat lamp (we had a chicken coop burn down when I was young from one so I never want one)
Do I go out and fluff up the straw again and just hope for the best? How cold is too cold? I dont have a garage to put them into.
Please offer any advice you have. My anxiety is through the roof.

My ducks go in a 4x4 wooden "house"
Its literally 4 pallets, on a Plastic pallet. The roof is galvanized tin. Vented at top with a plastic gutter turned upside down..Maple leaves are stuffed in the spaces of pallets. Floor is some rubber mud flaps with leaves and pine needles. They go in at night and i shut the door..they are let out at first light..into a free range settin. About an acre though they usually stay in a certain area free will...There are 10. No auxillary heat source. No lamp..we have had - 25 to -35 F ..they have always been fine..tjey hundle together..but i allow no water in the house..they have a pond..they go in if they want but usually sit ON the pond..However if you are worried bring them in..peace of mind but i would be worried then tbey WONT be able to handle the cold..perhaps put a house within their house? So they have a smaller place to gather heat...a couple picks..of inside amd out...
Safety first..and again, peace of mind.