I had 6 laying hens. 2-3 years old. They showed no signs of illness, were eating and drinking regularly, and didn't seem off or lethargic. No signs of parasites as far as I could tell. Out of nowhere, they all dropped dead, one by one, over the course of a few weeks. I lost my last 2 the other night, found them laying in a corner, one on top of the other, in their chicken run. I tried figuring out what was going on over this time period, but they just kept dropping. What could have caused this? And what can I do to prevent this from happening again before I get more chicks later this month?