Bone marrow is composed of mesenchymal and hematopoietic stem cells. It is found in the bones of mammals, reptiles, and amphibians. It is the primary site for the production of blood cells or hematopoiesis. Red blood cells (RBCs), platelets, and white blood cells (WBCs) are formed in the bone marrow of amniote vertebrates.
However, bone marrow is absent in the Aves or birds. The bone of the birds is modified to adopt flight. Most of the bones in birds are pneumatic in nature and filled with air sacs and replace bone marrow. In birds, hematopoiesis occurs in the bursa of Fabricius.