Today, I noticed that one of my ameraucana pullets has started to make strange sneezing / gurgling / wheezing noises, and when she makes the 'berk' noise that hens normaly make(as in "berk-berk-berk-urk") it sounds like Ber'cough'. She is still laying.

One of my silkies also has the same symptoms for the past 2 weeks.

Any ideas what could be the problem?

PS. It is the middle of winter here and it has been raining a lot lately so the run is muddy.

Also, I have 16 other hens that seem to be fine.