|MadSci Network: Zoology|
In birds, as in humans, red blood cells are made in the bone marrow. Birds do have "hollow" bones which are lighter for flight. They are lighter, however, because they have more cancellous bone and less cortical bone. It is in the cancellous bone, which is a honeycomb-like network criss-crossing the bone, where red blood cell production takes place. This network results in a very high surface area to maximize production of red cells.
You can check this for yourself by breaking open a leg bone next time you eat chicken. Chickens may not fly too much, but their bones are just like those in birds that do fly.
Note also that red blood cells in birds, unlike in humans and most mammals, still have a nucleus.
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