Teenage hormones kicking in. They are establishing who is top dog & who has the right to mate. If the 'brother' of the winner is appropriately submissive, the dominant cockerel may allow him to remain in the flock; if however the loser of this challenge refuses to submit, then the winner will effectively drive him out. This can manifest as anything between chasing the submissive bird, forcing him to hide from the alpha bird to death battles between them. Since this is occurring to younger cockerels, it's much more likely that they'll come to a truce. You need to try to ride this out, and separate the two when blood is drawn as the sight of blood stirs bloodlust in chickens (they'll keep picking at a bleeding wound, even unto death). If you need to separate them, put your bleeding boy into a wire cage and leave him in the coop so that the rival (and the rest of the flock) can still see and hear him to avoid him losing too much ground in the pecking order. A one-day scab of the wound is usually sufficient time before releasing him back into the flock to try to establish dominance again. This process can be short and sweet, or could drag on for a considerable amount of time... depends upon how determined each cockerel is in their goal to be 'the king'.Yes and they have been living together peacefully in the coop since June. There has been very little friction between either the roosters or the hens. This is a sudden change and I'm not sure if it's a coincidence that it happens during a period of bad weather where they have had to do with less space than normal, or if it would have occurred anyway. I haven't seen the smaller rooster come down from the roost all day.