a few days ago I noticed one of a hens chicks was lagging behind and seemed smaller than the rest. I picked it up and I could hear it making clicking sounds when it was breathing, and its breathing heavily (although not with the beak open). Its a few days later and there is no improvement, and today it also had pasty butt - something none of my chicks over the years has ever had. I cleaned it up. Its wings have just started to grow some feathers, but the wings seem floppy and hang down. The chicks are about one week old now. It does not seem to be suffering, and each time I pick it up to look at it I can't bring myself to end its life. Its feeding and drinking OK, and can keep up with the other - but also the last one. Do you think its got some disease that it will pass onto the other chicks and hens? I have not added any more chickens to the flock for over a year - so it can not have come from a new bird - but perhaps from wild birds like sparrows? What would you advise be?