Are you parking cars in the garage? If so the fumes could be making the birds sick. Also, if they have not been acclimated to the colder weather they could get sick from that. I would get your birds warm quick to help them fight what they are sick from. Possibly move them inside or get them ceramic heat lamps. Give them some vitamins in their water too. It is possible the new bird got the others sick. Most people quarantine their new birds before mixing them with their current birds.My birds are in garage which is covered, BUT IT IS LITTLE COLD.
Good luck