Is he trying to kill it? Or is he trying to mate with it?I am trying to figure out why a leghorn roo(he is huge) born at the same time (all 5 are 4 months old) is now trying to kill this bird.
If that bird is a pullet, and is getting ready to start laying eggs, the cockerel may be trying to mate with this pullet. Chickens mating can be rough, especially if the female does not want to, and the male is persistent.
If the cockerel keeps trying to mate with a particular female, she will often try to hide away from him.
All our other hens are not nice to him either. He sits on the back deck and is afraid of everything. Tried to put him in one of our other coops last eve and those 8 kept pecking him too. What the heck.
Once a chicken starts acting like a victim for any reason, other chickens are more likely to bully that chicken as well. So a pullet trying to get away from a cockerel can end up picked on by everyone else, just because of how she is acting.
(This is one possible explanation for the behaviors you have seen. I am not sure whether it is the correct explanation, or whether the correct explanation is really something else.)