Fallacy: bird bones are hollow

Is it true that bird bones are hollow? This might be a fallacy – a fallacy means that it is not true.

Bird bones are pneumatic. So are dentists’ drills and jackhammers. Vehicle air brakes and tyres are pneumatic. And vacuum pumps are pneumatic.

‘Pneuma’ is a Greek word for ‘wind’ or ‘breath.’ Pneumatic means that it contains, or is operated by, air or gas under pressure. It is compressed air or compressed gas. Air is a gas. Actually, air is a mixture of gases because it is 78% nitrogen and 21% oxygen, with the remaining 1% containing traces of water vapour, carbon dioxide, and argon. 

Ostrich bone

So, bird bones are not really hollow because ‘hollow’ means that a space has nothing in it, it is empty, or unfilled. 

Bird bones have pockets of air sacs with criss-crossing structures, like struts. These air sacs are connected to their respiratory system. With air-filled bones, birds can increase their oxygen levels to make their bodies create energy efficiently.

But not all bird bones are air-filled. Some bird bones contain bone marrow, which is tissue with blood cells. 

Birds that dive into water, such as auks, gannets, kingfishers, loons, penguins, puffins, and terns have solid, marrow-filled bones. Flightless land birds, such as emus and ostriches, have some air-filled bones. Birds that fly fast, such as swifts, have less air sacs in their bones than other birds.

Scientists in the 1600s first thought air-filled bird bones were delicate and lightweight relative to their body mass, but this is not true. Bird bones are dense, strong, and stiff. Therefore, the skeleton of a bird weighs about the same as a mammal of similar size.  

Bat researcher Elizabeth Dumont of the University of Massachusetts Amherst in America reported in March 2010 in the Proceedings of the Royal Society B that: ‘The fact that bird bones are denser than bones in mammals not only makes them heavier for their size, but it may also make them stiffer and stronger. This is a new way to think about how bird skeletons are specialized for flying and solves the riddle of why bird skeletons appear so lightweight and are still relatively heavy.’ 

She measured the density (mass and volume) of bones in song birds (small passerine birds), rodents (rats and mice), and bats. Density is a measure of mass per unit of volume – and denser bones are both heavier and stronger than less dense bones. She found that, ‘on average, the bones are densest in birds, followed closely by bats. Many other studies have shown that as bone density increases, so do bone stiffness and strength. Maximising stiffness and strength relative to weight are optimization strategies used in the design of strong and stiff, but lightweight, man-made airframes.’

Elizabeth Dumont says that over time, bird bones have evolved specializations that maximize stiffness and strength. The specializations include high bone density, a reduction in the total number of bones, fusion of some bones, and changes in bone shape. She says that the humerus, the main bone in the bird wing, is quite round in cross-section, which makes it stiffer in the same way that a round toothpick is harder to snap than a flat one.

Ostrich bone
Ostrich wing bone

Photographer: Martina Nicolls


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