How many younger ones? You are looking at maximum of 4 to maybe 5 birds, if everyone gets along, for that size coop. 4 sq ft per bird is really minimum.
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How sure are you that another chicken killed that one? Could it have been something else? Were they locked in that coop together when that chicken died or did they have access to the coop and the 10x10 run? Do you lock them in the coop at night or do you leave the pop door open?
How were they acting before this happened? Were the four keeping off to themselves and avoiding the older ones when they could? My pullets normally do that until they start laying eggs. Not always but normally. When they start laying they mature enough to force their way into the pecking order. When chickens fight the loser runs away. If it doesn’t run away the winner doesn’t know it won and it keeps attacking, usually going for the head. That’s where it can do the most damage.
This is purely a guess but it’s possible there was a pecking order fight and the pullet could not get away when it lost. If you leave the pop door open (that run would need to be predator proof to do that and you did not say you had a top) the loser could run away and tragedy would probably be avoided. This kind of stuff can possibly happen in an even larger coop but the more room you have the less likely it is to happen.
