MrsFrugal, if you are in the US, I suggest you call your county extension agent and see how to get a necropsy done. Find out about costs, how you should handle the bird if one dies, and where to take it. In some states it’s free, in some there is a charge. As Aart said if it happens again at least you would know what you are dealing with. This is good knowledge for any chicken keeper to have, let alone somebody trying to integrate new chickens. A necropsy is where an expert cuts the bird open to determine what killed it.
The reason you quarantine is to see if one of the new birds is sick before you expose your birds to them. You had a chicken die in quarantine. I’d be extremely nervous about allowing those new chickens anywhere near my flock.
Quarantine is not perfect. Some flocks develop flock immunities. They can be carriers of a disease but never show any symptoms no matter how long they are quarantined. When you put a bird that does not have immunity with them, the bird without immunity can get sick. What quarantine is mainly checking is if the new birds have been exposed to other birds recently and picked up a disease. If you got your new chickens from a chicken swap or some place where they are exposed to new chickens, quarantine is very valuable. As Aart said, it’s possible your flock is the one with flock immunity to something and infected the new chicken.
One of the very common flock immunities is coccidiosis. There are different strains of the bug that causes coccidiosis and immunity to one strain does not give immunity to all strains. Not all strains of coccidiosis cause blood in the poop but they all cause lethargy. It would be extremely easy for you to carry coccidiosis from one place to another on your shoes or a bucket that may be set on the ground. Coccidiosis thrives in wet conditions. If the area where you are keeping them has wet or they had dirty water, I’d be suspicious of Coccidiosis. Of course they may have brought it with them and it may not have come from your flock. And there could be many other explanations, another disease or maybe she ate a screw or small nail and it punctured her gizzard when the gizzard was grinding. Or many other things that are not a threat to your chickens at all.
What I suggest is that you pick a potentially sacrificial member of your current flock and put that chicken with the new birds. Then start quarantine over. Wait a month before they are let out to mingle. Base your decisions on what happens. Your quarantine may or may not have identified a serious problem but it has certainly raised a red flag. Good luck!
