I have 3 groups of chickens, group 1 is just over a year old, our initial flock of 13 chickens of about 6 different breeds. Group 2 consisted of 5 new chickens, of 3 different breeds. They are 3 months old. Group 3 consists of 6 chickens, these are about 3 weeks younger than group 2. Group 2 and 3 lived together in a brood area completely away from our primary flock. Our chicken area has a covered coop and an outside run area. 3 weeks ago we put group 2 in the outside area, there is a shelter for them out there. After a week or so, we let group 1 and 2 begin to intermingle under supervision. They seem to be fine as far as getting along. Thursday we took group 3 outside, and put them in the exterior area. Groups 1 and 2 were moved back under the covered shelter. Yesterday I noticed group 1 pecking at group 2, so I decided to move group 2 outside with group 3, since they are much closer in age i figured there would be less fighting between them. Today I went outside and found 5 dead chickens and 2 missing, though it's possible they got out through a gap that we did not see earlier. There are no injuries on them, from what I could tell. One of the heads was off, but there was no sign of blood around the body, so I'm guessing that happened after death. There's no sign of a predator trying to get in, there's not a bunch of feathers everywhere like they were being chased. Just 5 dead chickens. Any thoughts on what might have caused this?