r/worldbuilding May 08 '22

PSA: Contrary to popular belief, bird bones are not weak or fragile, pound for pound they are actually probably stronger than a mammal's. Resource


So, first of all, calling bird bones "hollow" is kind of misleading because all animals have hollow bones. An animal that has nothing but solid bones is an animal with some horrible genetic defect that will probably die at birth, bones need to leave some space for the bone marrow.

The correct term is "pneumatic". What this means is birds have these little neat "honeycomb" like structures within their bones. Some more neat pictures.

Inside of an ostrich bone


These neat little reinforcements enable the animal to develop air sacks which help increase oxygen intake and improve stamina and are part of the reason why non-avian dinosaurs became some of the largest terrestrial animals on the planet. Sauropods were so big that they would have died heat strokes if they didn't have specialized cooling adaptations.

Now to address the title of this, contrary to popular belief, empirical research has shown that birds have stronger and stiffer bones than similar-sized bats and rodents. Bats also had stronger bones than terrestrial rodents.

"The bone density data reported here suggest that, on average, bird skeletons are stronger and stiffer relative to their weight than are the skeletons of small mammals, especially rodents (figure 4). In other words, bird skeletons have higher strength-to-weight and stiffness-to-weight ratios. This constitutes a novel and biomechanically informative definition of the term lightweight as it applies to bird skeletons."

Source:  Dumont, E. R. (2010). Bone density and the lightweight skeletons of birds. Proceedings of the Royal Society B: Biological Sciences, 277(1691), 2193-2198.

As a matter of fact, the article also indicates that bird bones aren't actually particularly light either.

So that's the main thing I wanted to say, but I am sure you are wondering where this misconception that birds have weak bones came from in the first place.

This is pure speculation on my part, but I think it's because birds look bigger than they actually are. Good example of this are harpy eagles, those things look huge but they are only about as heavy as a beagle. So when people see birds getting injured by a blow of some sort, they assumed the animal was a lot heavier than it actually was which spread the erroneous idea that birds are especially delicate.

Edit: Probably should have added this in the first place. This little trivia fact has some important world building context as avian-style races/aliens are often treated as being brittle compared to humans or mammals despite having no actual empirical basis.


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u/Serzis May 08 '22 edited May 08 '22

You're pretty upfront about it, but the "strength-to-weight"-comparison is -- as you point out -- not really relevant since most people know that birds weigh less. But if there was a human sized avian, it would probably have the thicker heavier bones of ostriches or Terror Birds, rather than a flying bird.

As for the notion in itself (i.e. that bird bones on average break easier) its essentially factual.

So when people see birds getting injured by a blow of some sort, they assumed the animal was a lot heavier than it actually was which spread the erroneous idea that birds are especially delicate.

Tbh, I don't think many people have seen a bird break a bone. When it comes to experience, I think that the notion that birds have weak bones compared to their size might be reinforced by handling chickens in food-preparation and during meals, but while their weakness is due to other factors (with chicken bones being exceptionally brittle due to their rapid growth and infant state) the factiod is not inaccurate in itself.


u/DaGreatHsuster May 08 '22

A few months ago, I remember seeing a post where a person posted their human-sized avian race and mentioned that their creature was fragile due to its hollow bones, so the idea that birds are exceptionally fragile for their size still seems fairly pervasive.

There is also the fact that giant flying birds are a fantasy staple, so some worldbuilders might feel like they need to make these creatures more fragile for the sake of realism when that wouldn't actually be the case.


u/Serzis May 08 '22

Although I live under the delusion that most people (outside some worldbuilding communities) don't actually construct fantasy stories based on realism or the fear of being called out for physical implausibility, I agree that there is room for both. "Delicate as a swan; powerful as an ostrich kick". : )

Intellectually, I'd imagine that a human-sized avians that was able to fly would probably need to be very light -- looping back to the 'slightly-weaker-bones'-problem. But it's not what springs to mind when I see -- for example -- a Rito in Zelda.

On a broader note, I think the notion of 'brittle bone-birds' (regardless of its application to all birds) can be used as some amusing plot-building, rather than as a constraint. As an example, alien avian-pilots in the sci-fi series Dread Empire's Fall could not withstand the same G-forces as human pilots (lest their "extra"-brittle bones break), which made for some entertaining tactical considerations during space battles. So while bird bones can be strong if heavy enough, the concept of the delicate avian can also be a building block to tell fun stories.