Agree with both posters thus far....let them out...but as you said the silly birds WILL stay in when they don't want to brave the chill. (Mine absolutely HATE the snow.)
One word of warning on potting soil...make sure it does not contain any fertilizer beads or chemical additives in it as some do...that would NOT be healthy for your birds.
A really cheap source of dusting material, if you have one, is your fireplace ash...mine love to dust themselves with that and it is a very effective deterrent for the mites/lice...also add in some Sevin or Poultry Dust powder to boost the cleaning effect as cooped birds can get an overload quickly.
Then hopefully they'll go out and dust and play in the yard when the weather is better...I've actually had to buy a little "kiddie pool" (on sale CHEAP at the end of summer) as in my rainy Oregon, the availability of dust can be slim to none in our endless rain....I placed it under an awning of the shed...the girls line up and have dust parties now. I fill it with soil, ash, and sprinkle some Sevin in it. That has really help keep down the feather pests.
Lady of McCamley