OK I thought you saw blood on vent and didn't know where it came from And u just figured they were picking each other, welll thats clear now so check on the pellets u are giving them maybe that's causing diarrhea and they're dying because of dehydration. Are the ones you have now still with diarrhea? If yes give them pedialyte so they won't dehydrate. What color is their poop?They are not bleeding from the vent. They have diarrhea. They are bleeding from where the other chickens pecked at their vents. From what I see there is no blood in their stool.