Link #39: The Bones of Birds Are Hollow!

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Link #39: The Bones of Birds Are Hollow!

Squelette oiseau
A stylised bird skeleton 1. Skull, 2. Cervical vertebrae, 3. Furcula, 4. Coracoid, 5. Uncinate process, 6. Keel, 7. Patella, 8. Tarsometatarsus, 9. Digits, 10. Tibiotarsus (10 and 11), 11. Tibiotarsus (10 and 11), 12. Femur, 13. Ischium (innominate bone), 14. Pubi (innominate bone), 15. Illium (innominate bone), 16. Caudal vertebrae, 17. Pygostyle, 18. Synsacrum, 19. Scapula, 20. Lumbar vertebrae, 21. Humerus, 22. Ulna, 23. Radius, 24. Carpus, 25. Metacarpus, 26. Digits, 27. Alula.
Español: Esqueleto de un ave: 1. Cráneo, 2. Vértebras cervicales, 3. Fúrcula (clavículas), 4. Coracoides, 5. Procesos uncinados de costillas, 6. Quilla, 7. Rótula, 8. Tarsometatarso, 9. Dedos, 10. Tibia (tibiotarso), 11. Peroné (o fíbula), 12. Fémur, 13. Isquion (hueso innominado), 14. Pubis (hueso innominado), 15. Ilion (hueso innominado), 16. Vértebras caudales, 17. Pigóstilo, 18. Sinsacro, 19. Escápula, 20. Vértebras lumbares, 21. Húmero, 22. Cúbito (o ulna), 23. Radio, 24. Carpo, 25. Metacarpo, 26. Dedos, 27. Álula. Image credit: Mario modesto cc2.5

If you really sit and think about it, birds have virtually everything in the animal kingdom. They can go anywhere they want which means getting food is easy when compared to rodents of the same size. Even the Tibetans give them their dead bodies to feed on as we explained in our last post.

Most importantly, birds can fly! This also allows them to escape from most land-based predators. However, have you ever wondered what strange evolutionary branch resulted in birds being able to fly? In simple terms, birds can fly because their bones are hollow!

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Do Birds Really Have Hollow Bones?

Skulls of the Dodo and Rodrigues Solitaire, the latter having been scaled up for comparison. Image credit: Mr. Ford and J. Erxleben

The primary reason birds can fly is because their bones are hollow. You might think that this means that their bones are lighter. In fact, most teachers and scientists thought so too, until recently.

While birds’ bones may be hollow and it may be easy to think that this makes their bones lighter, this isn’t exactly true. In fact, the weight of birds’ bones is no less than that of similar-sized land dwelling rodents.

How can birds’ bones be hollow and still be the same weight as other animals of the same size? The reason for this is that birds boast of a greater bone density. This means that they have more bone cells in the same amount of space than other animals.

The higher bone density of birds combined with the fact that their bones are hollow makes their skeletal structure longer and stiffer when compared to the bones of other organisms, such as mammals.

There is another element to birds’ bones. This is the fact that many birds have something called struts and trusses inside their bones to give them the strength that they need. This means that birds’ bones have air sacs in them too.

How Do Birds Fly?

Bird Airsacs
Air-sacs and their distribution. Image credit: William Plane Pycraft, 1868-1942

The air sacs inside many birds’ hollow bones are often referred to as respiratory air sacs. These air sacs are said to have evolved at a time when the oxygen levels in earth’s atmosphere were much lower than they are now.

These air sacs helped the birds get more oxygen than other animals from the environment. It is also speculated that these air sacs help in flight by giving birds more buoyancy.

Why Do Birds Have Hollow Bones?

Diagram of skeletal structure and musculature of a bird’s wing, in color. The supracoracoideus works using a pulley like system to lift the wing while the pectorals provide the powerful downstroke. Image credit: L. Shyamal cc2.5

The big question is why birds have hollow bones. There are many reasons for this but the most obvious is flight. Birds have extreme adaptations in their bodies to help them fly. One of these adaptations is hollow bones.

The logic behind hollow bones is that they mean that the bird will not need as much energy as other organisms of the same size for going through their normal processes. Since flying is a highly energy intensive exercise, hollow bones end up supplementing that process.

Thus, you can say that birds have hollow bones because it helps them fly. Secondly, the air sacs that exist in the hollow portions of birds’ bones were first developed to help them breathe better in an environment where oxygen was scarce.

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  1. “How can birds’ bones be hollow and still be the same weight as other animals of the same size?”

    Same “size”? Lies, damn lies and statistics.

    Your statement is either selling out the facts for some shock factor, or you really didn’t understand what was said here.

    The issue is: Bird bones are LIGHTER for a comparable BENDING MOMENT. Period. Engineers and everyone else who knows stuff calls this an optimized structure. Bone density is simply how the optimization was achieved.

    Bird bones are still lighter though – for the same strength.


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