I have around 50 chickens and all have been perfectly healthy until a couple months ago. I had two barred rocks and four buff orpingtons give to me a year ago. One of the barred rocks was found dead two months ago, and then one of my buffs got a pale comb and started looking very sickly and then died a few days later. Now a third hen is acting the same way. It is only the hens that we're give to me last year and I have no idea what could be wrong with them that would take a year to show up. Anyone have any ideas or advice as to what could possibly be wrong with them? They have all been treated for worms and mites so I'm pretty sure it's not that.