My Chicken is getting Hurt

[Browns Farm]

I have 3 chickens who have been together almost their whole life's!!!! About 2 years and all of a sudden they are pecking the crap out of the ones chickens head and all feathers are gone! Omg what should I do????? Poor thing!!!!!!! Help?????????

 

[ChickenLover200]

Is there something wrong with the hen? Sometimes if others know a chicken is sick, they will intentionally cull the sick bird by pecking.