Are you changing anything this time of year like blocking up ventilation/closing windows? How are the ammonia levels inside your coop?
Have you ever treated them before? You mention this occurs every year, so what have you done in previous years? If they seem to be able to overcome the symptoms on their own, to me that would be what I would see if they are able to do this year as well.
Just a guess, but likely you have a respiratory disease in your flock. Birds remain carriers for life and can become symptomatic periodically. Changes in weather, stress, etc. can sometimes cause symptoms. There are several common diseases, without testing it's hard to know which one you are dealing with.
The smelly around the faces is a bit concerning though - do you notice mucous from the nostrils or patches inside the beaks? Watery/foamy eyes, facial swelling, etc.?
Some treat symptoms of disease like Mycoplasma with an antibiotic like Tylosin, you may be able to find it online. If you are dealing with a virus, then the antibiotic may help with secondary infection.