I have 12 Thai chickens just shy of 5 months, all sibling and been together since they've hatched. 5 roos and 7 girls. Yesterday one of my girls was cornered while her brother was biting her on the back of the neck drawing blood. I removed my girl from the flock for the day and after they went to roost, I swapped out the aggressive brother and put my girl back in with her family. Today the same girl was again cornered by a different brother that was biting her on the back of the neck. We again pulled her from the flock. We still have her separated. Does anyone have any idea of why she is getting attacked? No other girl is getting this treatment from others. My first thought was may be she is "coming of age". But, that's just a guess.