We have 5 12-week old Barred Rock girls. We originally got 7, and 2 went to live down the block with my neighbor and his 2 year-old RIRs. Last night one of his BRs mysteriously died, having had no sign or symptom save a bit of lethargy all day- both girls stayed hunkered down most of the day. It hasn't been hot, and he's been keeping the two pairs separate, so no bullying has occurred. Now the one BR is all alone, and we've become attached to the 5 we have, so we can't give one up, and he's inclined to have the chick come live here so she's with her pals (they only went to live with him 5 weeks ago) but I'm afraid of what could be brought back to my flock if we don't know the cause of death. What could kill a perfectly healthy 12-week old with almost no symptom? I'm so worried about the remaining girl, but don't want to risk mine! Thanks!