This might not apply to chickens but we noticed blood on our cockatiel a few months back and when we checked her over it turned out she had broken a blood feather. They can bleed quite a lot and can be a bit dangerous. Luckily in her case the bleeding had already stopped. Was just a bit of dried blood on her.

Could it be that?? Have they been moulting recently cos thats when you normally get the new blood feathers.