What diseases can chickens catch from wild birds?

So I have a flock in my backyard which have not been exposed to any other chickens in months. My two black cochins have come down with cold-like symptoms (sneezing, coughing, gurgling, crusty mucus in nose). I started Duramycin 4 days ago when the symptoms first appeared in one bird, but they still have the symptoms. They don't seem worse, and they are still eating and drinking normally, but the mucus, etc., persists. I think whatever they have must have come from the birds in my backyard. Does this sound like avian flu? What else could they get from the plethora of wild birds that visit my yard? I can't think of where else they'd get it.

A little more info in case anyone has words of advice: the cochins are 22 weeks (not laying yet). I've had them since they were chicks. I have 9 pullets/hens total, various breeds, only the cochins have the symptoms. The symptoms started 5 days ago, i started treating them 4 days ago. I have powdered Duramycin, I put 1 TBS in their water each day, and change the water daily. I feed them layer feed (Layena I think) and give BOSS, mealworms, and occasionally fruit/veggies from the garden for snacks.

Thanks!