My chicks have been amazingly healthy with no problems.They are almost 11 weeks old.I live in a mild climate area and until this week it's been warm.My hubby installed a forced air heater to keep the coop around 60-65 in extremely cold weather,like last night.That would be a typical temp for around here. This morning he checked the chicks early and everyone was up and active.I went out around 10 with more food and some cabbage and sand grit.I put that down in the coop and everyone ran to it.As I left I opened the hatch to the outside run. I went to fill up a waterer and to get some food for the run .I came back in through the run door and all the chicks came out to see what I had.I set it in place and sat down to watch for awhile. Right away I noticed my bigger rooster sat down in a box we have turned on it's side [just a place for them to feel safe]. I went over and picked him up and sat him down.He laid down and a second later put his head down.He was not gasping .His mouth slightly opened once,but he was very weak.Less than 10 minutes earlier he was very active. I put him in a tote and carried him inside .Two minutes later he was dead.No pasty butt,no odd breathing. I ran out and raked through the litter. All poop looks normal. I removed the cabbage and grit,although it didn't look as though much had been touched. Since then I have been watching the rest of my little flock.They seem fine. What could have happened? Could a sudden temp change from a warmer coop to the outside run have caused something that quick? We are only using the heater because they aren't fully feathered yet.