How birds produce red blood cells with hollow bones?

Inspiration

I was looking at a bird and I thought of the common fact that bird’s bones are hollow which is what allows them to be light enough to fly. It then occurred to me that humans produce red blood cells from their bone marrow, so where do bird produce their red blood cells?

My guess before digging in

Perhaps birds still produce red blood cells in their bone marrow, but possibly with a less dense structure, or it’s along the walls of the bone and not filling the inner bone structure, adding weight.

Findings

• Hematopoiesis is the formation of blood cellular components
• Erythrocytes are red blood cells
• Birds have bone marrow, but not in every bone, and not every bone in a bird’s skeleton is hollow. Their bones aren’t as dense as mammalian bones
• Human embryos produce blood cells in the yolk sac, then the thymus and liver during the majority of fetal development, and bone marrow takes over just before birth and in the early days of life.
• The bursa of Fabricius is the site of hematopoiesis and is a specialised organ that’s necessary for B cell development in birds. Mammals generally don’t have an equivalent organ
• Source and source

Etymology

• Erythrocyte
  • From Ancient Greek erythros ‘red’ and kytos ‘hollow vessel’, with -cyte translated as ‘cell’ in modern usage

Follow-up questions

How do some bird bones being pneumaticized work?

• Air sacs are spaces within an organism where there is the constant presence of air. among modern animals, birds possess the most air sacs (9-11)
• Birds have a system of air sacs in their ventilation system
• The hollow air spaces in bird bones outside of the head are connected to the air sacs in a way that a bird with a blocked windpipe and a bone broke in a manner where the inside of the bone was connected to the outside world could still breathe
• Some birds, such as penguins and loons, have solid bones

Why would bird RBCs be nucleated if ours aren’t?

• We don’t know. There are only theories