I've only read about this, but if you want to determine is something is blood vs. something else (like intestinal lining), you can pour some peroxide on the stool and it'll bubble away the blood, leaving the other stuff behind.
EDITED TO ADD:
- Have you done a physical exam of the birds - both deceased and alive? If they're underweight, you'll be able to tell when you touch them. Feathers hide a lot of physical issues so a visual inspection won't do any good.
- Have you noticed any bugs like mites or lice? Northern Fowl mites live on the bird and they look like moving specks of dust. You'll feel them on you. Red mites live in the coop and feed on the birds at night.
- If you are treating with Corrid, do not give any vitamins. Corrid can cause funky looking poops.