At first just one or two but now several of my chickens are "sneezing" and wheezing, sometimes shaking their heads. It seems this has come about because they are now spending much more time in the henhouse (I live in VT...and it's been cold) and, I will be honest here, they are overcrowded in there. I've been working hard to keep it clean and have recently purchased a second henhouse to accomodate the number of hens, but I'm not going to be able to relocate them fully for another couple of weeks, and I'm afraid left untreated whatever they have could get worse, claim lives even. In the past my vet has prescribed something on a case-by-case scenario (this has happened before, but only ever to one bird before I treated it and it went away) but he never did share his thoughts on what he thought it was. I'm looking to you, the chicken experts, for help. I'd like to know what this could be so I could research it, treat it and then develop preventative measures for warding off future problems. Thanks, in advance, for your help!