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How would a four-legged bird's flight differ from a regular two-legged one?
Would it be slower in flight but have more power taking off?
The bird in question is modeled on a bird of prey. (four legs of equal size and two wings)

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    $\begingroup$ Are we talking four legs and two wings, or some form of quadrupedal bird like bat or pterosaur? $\endgroup$ – John Jan 27 '18 at 5:36
  • $\begingroup$ four legs two wings $\endgroup$ – Ajnatorix Zersolar Jan 27 '18 at 13:35
  • $\begingroup$ You should really add that to the question it is important. $\endgroup$ – John Jan 29 '18 at 15:05
  • $\begingroup$ Depends on the size of the two extra legs - e.g. they could have T-Rex-like small front legs which they can tuck into their feather during flight - though I suppose they are more arms than legs... $\endgroup$ – colmde Jan 29 '18 at 15:58
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Any flying bird is aerodynamic. We designed planes from looking at birds, so it makes sense to consider the following:

  • Weight
  • Aerodynamics
  • Purpose

Weight

Adding an extra two legs to a bird is going to add weight. Which could impact is flight capabilities depending on leg thickness and size of bird. A griffon would have more chunky legs versus a pigeon, meaning more weight is being added.

A bird of prey should not have any real trouble with weight since their legs are very small anyway.

Aerodynamics

Here we can see a very elegant eagle, and a bird of prey.

elegant

Let's say we add two more legs, where do the legs go? If they go at the front you can clearly see that the bird would lose aerodynamic efficiency, adding them side by side with its current legs most likely wouldn't effect it at all, however I don't see this providing any benefit whatsoever should this be the case. This leads on to purpose.

Purpose

Eagles, owls and all predatory birds hunt using their legs or feet. Adding two more legs could help improve efficiency when hunting but at the same time it could hinder the bird.

For example, an eagle will use two feet, it only has one chance to grab its prey, add another two legs, it now has twice the chance to grab. For the better it becomes easier, but the bird would become lazier and wouldn't rely on precision as much as its two legged friend.

Whether this is a benefit for your scenario then great, but as a general answer to your question, I don't believe it will affect the bird much, although it would not really serve a real purpose.

Unless the bird in question was a mythical creature like a griffon or a hippogriff where its size requires having more than two legs.

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Most vertibrate life on earth follows a 4 limb body plan (in this instance, the wings are two limbs and the legs are the other two).
There are a number of mythical creatures with 6 limbs; Pegasus, griffons, dragons, etc.

One way to get 4 legs and wings is to have the wings double as legs. Something like the Quetzalcoatlus, which probably used it's wings as extra feet for instance.
Quetzalcoatlus

But you probably wouldn't want to ride these for long distances on the ground.

Really the only reason to have a flying mount is to fly. A ground animal can carry more weight. It can haul a load. It can wear heavy armor.
If a creature can fly, having limited ground mobility isn't a bad trade. If you need to get somewhere quickly, take off. If you need to get somewhere and don't want to be seen in the air, take a horse.

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I like the idea of a six limbed body plan. If your thing has four legs and two wings it can be more of a hawk, where it uses it's wings to travel but hunts by pouncing from the air and then is basically a lion on the ground. Long jumps for chasing sure. Even powerful enough to carry plenty of weight by the air back to the nest after its made its kill - sure. But don't expect it to be agile or graceful, this is a land predator that can fly.

Contrary-wise you can also have the falcon version of this animal that hunts other flying things (this is an and, not an either or) that has four wings and two stubby talons for ripping up whatever it can catch. This thing would be very fast and maneuverable in the air.

You are looking at creating a whole new taxonomy of griffin like creatures. There's a lot of room here to design what you like.

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    $\begingroup$ Six limbs gives you the potential for four winged birds as well. $\endgroup$ – OhkaBaka Jan 29 '18 at 17:03
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It would depend on the legs, or not.

If an otherwise normally flighted bird has a tiny pair of Tyranosaurus arms, it isn't going to make much difference one way or the other.

If they have a full on second set of drumsticks, they are going to be heavier and not fly as well as if they weren't carrying the extra weight.

Finally... look at existing birds with two legs... some of them take off gracefully and strongly with their legs kicking off under them... others don't use their legs at all... others have to get a running start... still others bounce like beach balls when they attempt to land.

Nature being what it is... 2 more legs can be anything you want it to be if your narrative is thoughtful enough.

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The only flying creatures with more than 2 legs are insects like bees or wasps.

Considering that the take off is an minuscule fraction of the time dedicated to flight, I don't see a clear advantage in having an additional pair of legs.

There might be a small advantage in having more grip on impervious area, but that would be probably overcome by the additional volume needed.

It may be different if the birds are non flying, like emus or ostriches. But in that case it would be better to evolve the unused wings to something like arms, for searching food in the ground or all those funny stuff primates do.

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  • $\begingroup$ I thought about giving them an extra pair of legs to make them more maneuverable and practical on the ground as well as in the air. The creatures are going to be used in battle and as a transport method so this seemed like a good idea. $\endgroup$ – Ajnatorix Zersolar Jan 26 '18 at 17:23
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To answer the first part, only really at take-off and landing since coordinating six limbs is going to be a more complex task than four. Take-off and even more-so landing is going to be restricted to spaces where this bird can get all four feet on a fairly even keel, no landing on thin branches for this critter.

As to flight speed that's about what the bird is designed to do, to be competitive as a bird of prey it needs to be as fast as a modern bird with the same specialty, whether it's a vulture with huge flat gliding wings or a falcon that relies on speed and precision it will have those same characteristics. Whatever it does it is almost certainly going to need a proportionally larger wingspan at a given mass than a two legged bird though because four legs are going to produce more aerodynamic drag.

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In my opinion, such a bird wouldn't be particularly maneuverable due to its center of mass being farther forward than ordinary avians. The difference in take off is negligible as well, sense it would just end up using its back legs to kick off the ground.

However, with two pairs of talons, this birdie could be amazing at climbing and landing on vertical surfaces. Not only that, but they would be able to grab and eat prey without risking a fall. Or maybe they could ambush landing birds, kicking off their perches while ensnaring their prey at the same time. If there spines are flexible enough, they could hunt nimble creatures that use trees as cover, like squirrels or monkeys.

However, regardless of their size, I don't think these hypothetical birds would make good mounts. In order to fly, they would have to have light-weight bones, which can't really support the weight of a human. Of course, a well done aerial combat scene or description of a flying caravan would make me too enraptured with awe to care about bird bones.

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