if they all came from the same place and all started with problems from there, could be from thme. But sounds like it may have been a while after before they got this. If so, check for rats and mice in your building, pens, and especially around the feed. They have been responsible for many many bird deaths, ATB isnt the only thing they can carry, but from your description that sounds like what you have in them. Try to get rid of the rats and the problem will go away for other birds.
