Pecking Order

Chickens are flock animal.However, chickens are territorial and they will fight to be leader of the flock. Chickens have always done this, some only come off with minor wounds and some have been pecked to death. But usually it takes an hour for the pecking order to establish, this is why chicken breeders recommend too buy birds that are the same size and age.
If there is a severe problem with the pecking order leave the pecked chicken within sight of others but out of reach.
This is also why I suggest that perches are the same height because otherwise they would fight over the highest one.
unfortunately, one blood has been drawn the 'peckers' become encouraged and develop a taste. Birds are particularly vulnerable to being bullied when moulting or after an accident. If this problem begins to look excessive try some 'anti-pecking' spray by Barrier which has an off putting smell/taste. If this fails, you may wish to separate the bird until she is recovered.