Cold, wet weather is conducive to respiratory diseases. If you're only hearing an occaisonal sneeze, it may be nothing to worry about. Just keep a close eye on them, and try to build up their immune system by giving som electrolytes and probiotics. Keep them out of drafts, if you can.
If you begin to hear more sneezing, or notice other respiratory symptoms like coughing, rattling, gurgling, swelling of the face, and discharge, I advise getting antibiotics. If you have a lot of birds to treat, a water soluable antibotic like a form of oxytetracyline would be a good idea. Oxytetracycline should be pretty easy to find, under names like Duramycin, Terramycin, and Tetroxy HCA-280. Keep in mind, though, that antibiotics will not help viral respiratory diseases.
One of the better antibiotics for respiratory diseases is Tylan50. It is usually given as an injection, but can be given orally, and there is even a powdered form that can be given in the drinking water. The injections are probably the best idea, though. To give Tylan as an injection, you'll need some small needles (20-22 gauge) and some small syringes. Give 1cc for large fowl or large birds, and about .5ccs for smaller birds like bantams. Inject it into the breast muscle once daily for five days. You should see improvement by the third day if it is working. Here is a link to giving chickens injections, in case you've never done it before:
http://shilala.homestead.com/injection.html
Keep their energy up by giving electrolytes and probiotics. Make sure that they get enough to eat and drink. Try giving some scrambled egg, yogurt, or applesauce. If you notice more signs of sickness, isolate the sick birds immediately.
Hope this helps!
