2004). B-lymphocytes, the cells that produce antibodies, are initially produced in the embryonic liver, yolk sac and bone marrow, then move through the blood to the bursa of Fabricius (BF). Birds have a bursa of Fabricius (BF), which is an outpocketing of cloaca.
Most squirrels are vegetarians. There are a few exceptions like the flying squirrel. The flying squirrel will eat small birds, insects and definitely steal eggs from birds nest in order to make a meal. Generally speaking though, squirrels will mainly stick to things that are vegetarian related.
Grey squirrels are mainly herbivorous, eating acorns and hazel nuts, berries, fungi and even bark, buds and shoots. However, on rare occasions when plant food is very scarce they will eat insects, smaller rodents, bird eggs and nestlings. Grey squirrels breed twice a year, December to February and May to June.