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This question already has an answer here:

If peregrine falcons, albatross, and hummingbirds were all human-sized, but possessing their current biologies, would each of them still be able to fly as they normally would?

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marked as duplicate by JohnWDailey, JBH, Mołot, bilbo_pingouin, elemtilas Dec 14 '18 at 7:38

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  • $\begingroup$ What dimension should be human sized, length or mass? Because the answers will be yes and no. $\endgroup$ – kingledion Dec 14 '18 at 3:14
  • $\begingroup$ Both legth and mass to make the size conversion equal and sensible. Basically if the height, weight/mass, wingspan were all scaled up to human size but without changing the body ratios, BMI, amount of muscle percentage, organs sizes in proportion to the body percentage space they take up, bone hollowness, etc being all the same as the original sized birds. $\endgroup$ – Arizona Tallent Dec 14 '18 at 3:29
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    $\begingroup$ It cannot be both. Mass proportionality goes with the cube of the length, if you double the length you are increasing the mass by 8 $\endgroup$ – L.Dutch Dec 14 '18 at 3:52
  • $\begingroup$ Also, birds weigh less by volume than do mammals, because of hollow bones, not to mention feathers. A 120 lb bird would be much much bigger than a 120 lb human. $\endgroup$ – Cyn Dec 14 '18 at 4:42
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    $\begingroup$ @elemtilas I concur from experience. Once I had a car break down in the middle of nowhere at night. No human habitation for miles around & no traffic. While we fixing the cat, an emu walked out of the darkness. They are as tall as a human. You can look them in the eye, noticing their very nasty beaks. Quite powerful brutes too. After walking around us to see what we were doing, it disappeared into the night. The memory still gives me a chill. Mid-sized dinosaurs, indeed! $\endgroup$ – a4android Dec 15 '18 at 3:50
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Nope.

Scaling up animals just doesn't work. You'll need to adjust proportions as things change. Notice that an elephant has very thick, stocky legs that go straight up and down, while insects have spindly legs that are off to the side. You can make big things fly, they just can't look like the little things.

This is due to a phenomena called the "Square-Cube Law." If you double something in linear scale, you quadruple it in area scale, and you octuple it in volume scale. Take a cube 1cm on the edges. It has side areas of 1cm2 (6cm2 total) and a volume of 1cm3.

If you double it in the linear scale, the edge lengths are now 2cm. This means each face is 4cm2 (24cm2 total) and the volume is 8cm3. The area went up by a factor of 4 (2*2) while the volume went up by a factor of 8 (2*2*2). Mass will scale with volume, unless you change materials to have a lower density.

That is the math behind the square cube law. Now, what does this mean for scaled-up birds?

Well, first off, you'll notice that mass is doubled per area when length is doubled. This means the pressure on anything supporting the object, like a wing or leg, scales with linear measures (aka length). This means that scaling up without adjusting proportions wouldn't work, since the increased force requirements to stay in the air are not countered by increased wing area.

Also, here's a thing about muscles - their force scales with cross-sectional area, not volume. So your scaled-up birds will have scaled-down force, relative to the masses of their body parts. Like wings. Not only would they not have enough force when flapping at their normal rate, but their muscles couldn't even handle flapping at that increased rate because of the increased mass.

The strength (amount of force they can take) of bones is also dependent on the cross-sectional area, so it'll scale down as you scale something up. If you scale things up too much, they'll collapse.

This is why big animals are so stocky: they need to have much more muscle, relatively speaking, than smaller animals because they need stronger bones and muscles, and the only way to do that is through making them bigger. (Well, there are other ways, but they come with their own tradeoffs.)

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A bird is able to fly because of its unique body structure. Hollow bones, specialized feathers, and sometimes specialized bladders all work to ensure that each species of bird has the ability to propel itself into the air. The scaling of these structures is possible, case in point being the California Condor, which is far larger than the human-sized birds you are talking about.

Not all birds are able to be scaled up in such a manner, though. The California Condor is only able to fly for long periods of time because of the gliding potential its huge wings grant it, as is the case with most large birds of prey and seabirds. But songbirds and hummingbirds, which use near constant wing motion to fly, would not fare as well. The increased mass of their wings would make flapping them too energy inefficient, and their body and wing structures are not the right proportions for them to attempt to use gliding as a work-around. Plus, their hearts tend to beat fairly quickly, which could lead to serious heart failure if they were to be scaled up.

So, birds such as falcons, eagles, albatrosses, seagulls, etc. would likely be able to function at a human size, but smaller birds, such as hummingbirds, songbirds, parakeets, etc. would not be able to handle it.

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  • $\begingroup$ Indeed, a hummingbird wouldn't be able to fly if you scaled it up to the size of a crow, or even a starling. (Not to mention the problems it'd have feeding itself.) You occasionally see larger birds hover, but only for a few wing strokes, not for long periods. $\endgroup$ – jamesqf Dec 14 '18 at 5:40
  • $\begingroup$ "case in point being the California Condor, which is far larger than the human-sized birds you are talking about." At 11 to 15 kg total mass, you might want to think again. Even in terms of body length (beak to tail, including the tail feather length) condors are shorter than male humans - max of about 53 inches), scaling up to human length would make them too heavy. $\endgroup$ – WhatRoughBeast Dec 14 '18 at 6:24
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Quick answer is nope.

I think its due to how muscles work and how they are more efficient at lower weights (a baby can be lifted with just a finger, try that with me after I've been in bed for 9 months and I would barely be able to lift up my hand).

After a certain size, your muscles become to heavy, your bones become heavier to support it all and you just can't lift off. You can look at the Emu and Ostrich as examples, and even penguins can grow to be pretty large. Heck.... even chickens have problems flying.

The long answer... is up to someone else with more knowledge.

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