Can you post some photos?
How much coop space do you have?
How much run space do you have?
How old are your chickens?
If they are old enough to lay eggs,when was the last time they laid eggs?
- A bloody vent can be caused by a few things.
- Feather picking - other chickens plucking/pulling the feathers out of each other can cause blood.
- A prolapsed vent can look/be bloody - this is where the oviduct/intestinal tissue is has come out of the vent -chickens will pick at this causing injury.
- A predator attack can also cause injury/wounds.
Calcium deficiency would generally NOT cause a bloody vent. But Calcium is important to a hen's diet. Feed them a nutritionally balance poultry feed with oyster shell provided free choice.
If you haven't done so, wash your girl's behinds and inspect their vent to see if you can tell where the blood is coming from and what possibly caused it - picking, prolapse, attack, etc.
Let us know how they are doing.