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Sometimes I reminisce about the giant eagles from The Lord of the Rings and two things come to mind:

  1. Boy, I wish I could have my own giant eagle!

  2. Could a bird even carry something that heavy on its back?

How big would a bird have to be in order to carry an average human being on its back? Are we talking roc-size or something smaller? Would it be able to wear some sort of harness so its passenger(s) weren't merely clinging on for dear life? And won't someone make these hypothetical giant birds real so I can commute to work in style?

EDIT I left this question to bake overnight and I have not been disappointed! Anyway, someone asked me to specify whether the person could be carried on the ground or flying, and I intended for both person and bird to be in the air at regular bird altitudes.

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Well, we already know that a five ounce bird could not carry a one pound coconut. – cobaltduck Mar 21 at 20:44
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Unfortunately, giant eagles cannot carry even a small humanoid (say a hobbit carrying a ring) for any appreciable distance, certainly not to a distant volcano. – Cyrus Mar 21 at 21:38
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Golden eagle snatches kid? youtube.com/watch?v=CE0Q904gtMI... i guess this eagle should have been at least twice its size to make it – Erik vanDoren Mar 21 at 21:43
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Note to all answerers (especially new users!): Please look at the hard-science tag wiki for information about what an answer to a question with the tag should include. The tag indicates that answers should strive to be a scientifically sound as possible; this doesn't mean just linking to web pages, but to reputable scientific papers or other similar evidence. Thanks! – HDE 226868 Mar 21 at 21:46
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@ErikvanDoren props on the video and I voted you up for it, but unfortunately (or, perhaps fortunately), it's a hoax. Says so in the description. – userLTK Mar 22 at 5:08
up vote 41 down vote accepted

I decided to try to extrapolate from some known data. I used various sources to find that:

Using just the harpy eagle and peregrine falcon data, the answer to "How much can a bird lift?" appears to be "About half its own weight." I admit my sample size is very small, but sounds reasonable for a first round estimate.

However, we also have to understand that the carrying capacity of an animal follows a rule of diminishing marginal returns. A 5 mg ant can hoist a 500 mg leaf (10,000%), while a 5000 kg elephant might be able to carry 500 kg of logs (10%). The larger animal can carry more, but at a significantly reduced ratio. Our ever larger birds might be reduced to 30-40% or less.

That means to lift a husky human we need at least a 200+ kg bird. It seems that the known size range of birds falls very short of that. Even a gaunt human is almost equal in weight to its would-be feathered steed. Sorry.

Meanwhile, if we expand to pterosaurs and their ilk, we might just have a chance. It is becoming my go-to example. Assuming the higher-end estimates for its mass and a generous allowance for its lifting capacity, I would love a ride on a Quetzalcoatlus. (Do I have to say it yet again? Modern literature needs more Quetzalcoatlus.)


Update: Since posting this a month ago, we now have this. I am tempted to recant my entire answer in favor of Mr. Munroe's.

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You have to take into account that bigger animals are less efficient. So a 200kg eagle won't lift a person and carry them over a long distance. A 200kg hummingbird won't hover. – Mindwin Mar 22 at 12:27
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To note: As the wingspam increases the wing are increases by the square and the bird weight increases by the cube. This alone puts a limit on the size of a flying bird. Also note a bird of prey strikes the prey in a flyby. The weights you use in your calculations can be a lot more than the maximum take-off weight. – jean Mar 22 at 13:48
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A useful thing to know in cases like these is the "square/cube rule". It is a very rough measure that lets you extrapolate strength and carrying capacity from small creatures to large. Strength tends to vary according to muscle cross-section which is proportional to height squared. Weight varies according to height cubed for similarly shaped creatures. Again, rough measures. So if you magically made a swallow with twice the wingspan it would be four times as strong but weigh eight times as much. – Hugh Meyers Mar 22 at 13:50
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Square cube law is here: en.wikipedia.org/wiki/Square-cube_law. Should be required reading. :) – Hugh Meyers Mar 22 at 13:57
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But what is the airspeed velocity of an unladen swallow? – Anoplexian Mar 22 at 19:48

Yes. Let the image explain itself.

enter image description here

This file is licensed under the Creative Commons Attribution 2.0 Generic license.

According to the 1920 edition of Popular Science magazine, there was a tourist attraction in Florida that allowed passengers to ride an ostrich, and they determined that the carrying capacity be 150 pounds, which is realistic for a light human or a hobbit.

If you weigh more than 150 pounds, consider going to work by being pulled by the ostrich.

enter image description here

This media file is in the public domain in the United States.

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hard-science, please. Additionally, I suspect that the question was asking about carrying a human while flying, not walking. – HDE 226868 Mar 21 at 21:31
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You mention that the copyrights of the first image are CC-BY, yet you fail to attribute it. – bilbo_pingouin Mar 21 at 22:04
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@HDE226868 "All answers to this question should be backed up by ... empirical evidence" I think that requirement was satisfied. – PyRulez Mar 21 at 22:39
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@HDE226868 The OP technically didn't specify. Perhaps we need a standard loopholes list like code golf? – PyRulez Mar 21 at 22:43
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I did mean a flying bird, but since I didn't specify that, +1 to you for the answer nonetheless :) – jackwise Mar 22 at 14:23

No bird in current times can fly with a human on its back. Pelagornis or Argentavis (both extinct) might have been able to fly with a human on their backs. However they too, would need to come in flying at maximum speed, pick up a human in their talons and then continue with a lot of initial effort. The shock alone would be enough to break a few ribs or cause excessive internal shock damage which would have severe short and long term consequences.

Argentavis size comparison with an average human.
enter image description here

Pelagornis size comparison with an average human. enter image description here

Forgoing birds, a couple of pterosaurs might have been able to pick up a human more easily than the birds mentioned above. The two candidates which come to mind are Quetzelcoatlus and Hatzegopteryx.

Quetzelcoatlus size comparison with an average human.
enter image description here

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hard-science, please. Thanks! – HDE 226868 Mar 21 at 22:07
    
Also consider aerodynamic drag and weight & balance. – jamesqf Mar 22 at 5:20

I'm not sure about carrying something on its back. The lightweight bone structure found in most birds would seem to work against them as a beast of burden.

Instead I'll try answering as if the bird is carrying the person (directly or in somewhat less terrifying accommodations). A bird of prey can carry half its body weight, as a rough estimate. This sounds promising, but a condor, one of the largest birds by wingspan, weighs in at around 30 pounds. If we scale this up (not how biology actually works...) so that our bird can carry a 200 pound package, we get a bird with an approximately a 130 foot wingspan. That is a very large bird.

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Please note the requirements of the hard-science tag, which can be found in the tag wiki, and edit your answer a bit. Thanks! – HDE 226868 Mar 21 at 21:32

How big would a bird have to be? Simple - the same size as a dual hang-glider. A bird's wing and a hang-glider wing are solving basically the same problems, and with basically the same efficiency as a result. If the bird's body is about the same size as a human's, in order for it to haul its huge wings around, then we would need a bird with dual-hang-glider-sized wings in order to support a human. Some extinct birds or pterosaurs (see other answers) may have been about the right size.

The problem then is bone stresses. Flying endoskeletal things tend to have radically-lightened bone structures. Whether the wing bones could take the extra weight is very doubtful.

And unfortunately for LotR, Avatar, and every film or fantasy series showing people riding giant eagles, sitting on its back is probably a bad idea. The centre of balance goes above the centre of the wings, you become unstable, and you both plummet out of the sky. A hang-glider-style harness under the bird's body is much more practical, even if it looks less cool.

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A hang glider (aluminium hollow tubes and cloth) weighs around 100kg or less. A flesh-and-blood bird that size would be over 300kg. Not reasible. – Mindwin Mar 22 at 16:00
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@Mindwin Not true. Argentavis apparently was around 72kg, for a 7m wingspan, and pelagornis apparently was only 22kg with about the same wingspan. So those birds existed, and that's not an unreasonable wingspan of glider for people with a jockey-type build. The problem of course is that those birds achieved those weights with porous bones, so whether they could take the extra weight of a rider is more of an issue. – Graham Mar 22 at 18:08
    
This is probably the most well reasoned answer I've seen thus far. – 16807 Mar 22 at 21:46
    
There is no problem with CG being above the wings - the birds simply need to adjust attitude more often (maybe get tired faster) but will not be unstable. The only problem with stability is when the CG is behind the centre of lift (for airplanes that tend to be around 1/4 to 1/3 the distance from the leading edge, for flapping wings that tend to be in the middle of the wing). We humans with our slow reaction times manage to learn to fly low wing planes. It would be even less of a problem for a bird to learn to fly top-heavy. – slebetman Mar 23 at 9:07
    
@slebetman Fair point, it won't be aerodynamically unstable, but it will be twitchy as hell, and whether it's still controllable by the bird would be iffy. – Graham Mar 23 at 10:38

Just wanted to address something someone brought up:

"This thin-walled architecture has led to the common misconception that pterosaur skeletons were extremely lightweight. This myth fails to take into account the overall volume of the bones, which is actually quite large. The same trend (and associated misconception) applies to birds: many species have very hollow bones with air sacs inside (these connect to the respiratory tract), and yet the skeleton of a bird weighs the same as that of a mammal of the same total weight. In both birds and pterosaurs the thin walls of the bones act to increase the strength of the skeleton without adding additional weight"

source https://pterosaur.net/anatomy.php

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Welcome to the site, Alex Bv. I have upvoted your answer as a welcome gesture, but I suggest (this is just my thought) that you try to concentrate on answering the question which has been asked. Once you get some reputation points, you would be able to post comments and suggestions on other peoples' answers. Use that ability to post comments :) Until then, try to focus on the question which is asked. That would help to keep the answers from derailing from the main theme (the question) and stay focussed. Thanks. Please don't take offense from my comment. i am only trying to guide you :) – Youstay Igo Mar 23 at 6:49
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Actually, bird skeletons ARE extremely lightweight - leading to their total weight being light as well. While bird skeleton may weigh the same as a mammal of the same weight, bird skeleton weight significantly less than a mammal of the same SIZE (volume). The key to aviation is wing-loading, not total weight. This is true for airplanes as it is for birds. A toy plane with a 2m wingspan weighing 1kg is light. Another toy plane with a 50cm wingspan weighing 500g is heavy (it should be around 200g to be considered light). – slebetman Mar 23 at 9:13

I don't know how big a flying creature has to be to be able to carry the OP, but I don't think that a giant eagle makes a good form of transportation.

If you ride on the back, you are likely to be blown off if it goes fast enough, and the wind will certainly be uncomfortable and blow bugs in your face. The eagle could carry you in its claws but if it doesn't wrap them around you but sticks the talons in you you could bleed to death. There is a man alive today who claims a giant thunderbird tried to pick him up and fly away with him when he was a kid, and he claims he was terrified at the time.

And you still get the uncomfortable wind. And you might be in position to be pooped on.

You might have a saddle or harness or even an enclosed cabin like the pterodactyl airliners in The Flintstones but then you still have the problem of steering and controlling your presumably not supernaturally guided bird.

So I don't think that a giant eagle ride would be cool.

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Note that the question is not "is it a good idea?" but "how can it be possible?" – Hohmannfan Apr 1 at 17:19

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