Male territorial birds can be beyond aggressive if there's a dispute with a neighbouring competitor. E.g. the robin, the little bird we kinda see as a friendly/nice bird, can fight his rival till death in the mating period.
Also, the male blackbird in my garden looks like he's been in some quite severe fights too. (well, his rough/bit damaged appearance now can be caused by defending his offspring (or his lady) against magpies, crows, cats, etc. as well)