The bones of birds are not completely empty. They are lighter in weight and have air pockets. The bone marrow in birds, like that of other animals, produces red and white blood cells, but the location and distribution of the marrow is different due to the unique structure of bird bones. HSCs are primarily found in the bone marrow of birds, which is a spongy tissue found within the bones. However, some studies have suggested that HSCs may be present in other tissues in certain bird species, such as the liver and spleen.