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.
Squirrels do not attempt to eat any of the various birds here. You will see cardinals, doves, and blue jays eating side by side with squirrels some days. Just don't leave peanuts on the ground use a peanut feeder. Squirrels are very protective of nuts they will chase anyone away who attempts to have a bite.