Bone marrow.
The inside of human bones have called "bone marrow". In adults, marrow in large bones produces new blood cells. Source: Mah Brain.
Flight requires that the weight of the bird be kept as low as possible.
If you mean "light" as in "the opposite of being heavy", then: Bird bones are hollow. Well, not completely, but if you look at the structures they are not as "filled up" as, say, human bones.
No they are not empty. Our bones are filled with bone marrow.
the bones are filled with bone marrow
In addition to having a different arrangement, the bones of a pelican are hollow, which is a characteristic shared by all birds. The bones of humans and other mammals are not hollow but filled with marrow.
The answer is bones. Bones are filled with marrow and have air spaces to make them lighter in weight.
Human bones.
humming bird bones are hollow, while human bones are not holllow
marrow
bones
How do you remember the bones in the human body?