We have all had the experience of going outside in winter and seeing your breath. What you are seeing is described as insensible water loss. The cold air is freezing the amount of water in your breath that you exhale. That same air also carries viruses, bacteria and other substances. During the winter, when the homes are closed up, there are increased concentrations of viruses and bacteria in the air.
Most diseases are dose related.
The solution again is ventilating the aviary. W interizing your aviary means checking the heating system, supply fresh water, air and adequate humidity. Low temperature levels are not as harmful as great fluctuations. Good nutrition must always be maintained .