Hi, Oenwen, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. Meg has what is called colitis, which means that the lining of her large bowel or colon becomes inflamed periodically from "something". The most common cause is a dog eating a new or fouled food. Eating the dead birds could definitely be a cause, but eating a new food or table food could also lead to this inflammation which creates the blood or and/or mucus in the stool. Other causes can be parasites, bacterial and viral infections, food allergies. Since Meg recovers so quickly, it is probably due to her eating unusual things.