A small, glandular, blind pouch of lymphatic tissue, the bursa lies on the dorsal side of the cloaca. It is lined with endoderm, and opens into the proctodaeum. In a young bird, bursa fabricii forms lymphocytes, and probably it produces antibodies and protects against local infection, but it atrophies in the adult before sexual maturity. It is called cloacal thymus, because like thymus, it secretes lymphocytes. Thus it is not related with flight adaptation, rather it helps in immunity.