Sudden death

I have lost 2 of my ladies over the last 3 days, suddenly and with no apparent signs that there was anything wrong. The first was dead in the coup when we arrived home from work. All was well when we left. No apparent signs of injury. The second we found this morning, with physical signs that there was something wrong. She's sitting in the same place, won't move disinterested and her feathers look pale. The other two we have left look fine, just like she did yesterday but we are unsure how long it will be before we see the same in them. A very sad day, for us because there was no warning. We are trying to think of all sorts of things that could of been. Other birds, something they may have eaten, worming. We thought that with the shorter days of sunlight this may have contributed to less eggs, but it may be that sickness of some kind was the cause. Any information would be most welcome... they are all such healthy happy ladies, that free range. We have had them 5 months and never had any problems until now.