I believe you have given the dimensions of your run (pen). What is the size of the COOP (L x W x H), and can you describe what is in it for perch space, and location of the perch? (length, diameter, space between perch and back wall, ceiling and floor of the coop.)The pen is 5 m x 2.5 m. The coop can house 10-12 chicks. The aggressor is a speckledy hen. I have a blue bell, a black rock, a Orpington and a rir roo. I'm sure it will have happened in the coop overnight. The injured one has been seperated since i found the injury. It will stay that way till it's healed. I'm not sure if they are bored. I think it's just that I have an aggressive chick.
If you are stating the dimension of your run, that is too small for your existing birds. You have 12.5 sq. meters = 41 sq. feet in the run. That is big enough for 4 birds in the run, assuming they are all female. IMO, if you have a roo, you should double that amount of space. Your run needs to be big enough that there can be multiple levels (hay bales, stumps, perhaps a perch) so the birds can move out of the same plane as their aggressor, and get out of sight.
If I am misinterpreting your post, and the stated size is related to your coop, my apologies. Manufacturers often misrepresent their product, and while they state that their coop will hold 10 - 12 birds, that is not often the case, resulting in behavioral issues, cannibalism, and other difficulties for the poultry owner.