Chicken Lost to Unknown Illness

When I went out to close up the chicken coop last night I noticed that one chicken was missing.  A quick head count, and I realized it was the Barred Rock hen.  She's had a droopy, pale comb lately, and has been feeling poorly.  I fully expected to find her dead in the chicken yard.  All but.  Something neurological had happened to her.  She was on her side and unable to stand.  I gently picked her up and settled her into the isolation crate in the barn.  She was still alive today, but obviously terminal.  This afternoon, I convinced Rouse to perform the unpleasant job of putting her out of her misery.  I haven't killed a chicken yet, but one thing I know for sure.  If I'm going to be responsible for a bunch of chickens, I've got to learn how to cull the unhealthy from the flock.