I would guess you need to use their system. However let your birds out even in the winter. They are wearing down coats and will go into shelter if they feel the need. We raise ducks for meat, eggs and just to look at. The ornamentals are separate from the domestics, a couple of the domestics sleep with the geese, but most prefer to stick to their flock for bedtime. I dont know how many you have but a basic dog house shape with ventilation and a secure door for over nigh would allow you to keep both still and not worry as much about the size difference. I would be more worries about the mallards flying off and not returning (read often others going through this).