I know I’m late to this post but wanted to share my experience, which is very different to what others are posting. Chickens form strong social bonds and they do mourn when a flick mate passes if you pay attention to their behaviour.
I recently witnessed one of my hens pass and she actually walked towards her flock and passed with them watching. After she passed some appeared not to care but others sat on the spot where she passed and I would see them doing this for long periods of time for about a week. They also ate noticeably less and lost weight. It was really sad to see. On another occasion I had to rehome a rooster and the girls appeared to be looking for him for a few days. They would also call for him in the mornings.