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On a certain planet, there is a large population of falcon-like birds. They dominate the avian ecosystem. However, a population of nearly-identical birds soon finds its way in. The only difference between the two species is that members of the new population have four wings instead of two.

What advantages (if any) would the four-winged falcons have over the two-winged falcons? Would they be faster? Quieter? Better at maneuvering?

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    $\begingroup$ Quieter? Birds are pretty quiet when they keep their beaks shut... $\endgroup$ – Isaac Kotlicky Mar 24 '15 at 0:12
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    $\begingroup$ @IsaacKotlicky I remember reading something about the acute hearing of some rodents, which could make any tiny noise a problem. But I can't find a source for that. $\endgroup$ – HDE 226868 Mar 24 '15 at 0:15
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    $\begingroup$ @HDE226868, Owls are quiet fliers for exactly that reason $\endgroup$ – Monty Wild Mar 24 '15 at 0:21
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    $\begingroup$ Two wings bad, four wings good, huh. Would there be any interactions between the two species? $\endgroup$ – Linkyu Mar 24 '15 at 15:56
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    $\begingroup$ This made-me think of a X-Wing like falcon, closing it's wings together to increase speed or opening them on a X pattern to increase mobility. And this seemed pretty cool on my head. $\endgroup$ – T. Sar Mar 24 '15 at 17:46
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I'm going to have to disagree with Monty on this one. While the wings would be added weight and would potentially create more drag, comparing bird biology to biplanes is downright folly.

Consider that almost all flying insects use multiple pairs of wings to produce lift. Flies not only have ridiculous acrobatic ability due to their ability to swing their wings independently, they also move at a relatively high speed when measured in body lengths.

A dual wing bird alternately flapping pairs would more quickly gain altitude - while one pair is in the up-stroke, the other would be engaged in the lift generating down-stroke. When gliding or surfing an updraft, the extra pair of wings would provide much more lift like the delta wing of fighter jets and gliders. This gives them more maneuverability and a higher stall angle.

So not only would a four winged bird generate more consistent flapping power, it would be a better glider, climber, and diver. The trade off would be weight and noise - assuming they are noisy; owls are pretty quiet.

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    $\begingroup$ Unlike most insects, flies (order Diptera: en.wikipedia.org/wiki/Fly) actually have only two functional wings. The vestigal second pair have evolved into halteres, which serve as a balancing organ. $\endgroup$ – jamesqf Mar 24 '15 at 4:50
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    $\begingroup$ Owls are quiet due to the nature of their wings. Their wings are 'fluffier' than normal birds, which dampens the sound. Though I suppose four wings could decrease sound in that each wing would only have to work (approximately) half as hard, thus assumedly decreasing the amount of sound they would generate. One negative I see for having more wings is that the 'bird' would have to spend more energy using its extra wings. $\endgroup$ – Yay295 Mar 24 '15 at 5:44
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    $\begingroup$ I'll also add that some insects, particularly beetles, have two hardened wings that act a lot like plane wings and two regular wings that provide the thrust. Further, almost all the other insects with regular four wing compliments actually link the wings on each side and they mostly function as two, not four independent wings. $\endgroup$ – fredsbend Mar 24 '15 at 10:01
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    $\begingroup$ The physics of a four- or multiple-winged insect do not necessarily scale up to birds. The acrobatics of a fly, then, might not necessarily translate to a bird. In sum, if the world you're creating has similar physics and you care about accuracy in that regard, you might be interested in what Biology or Physics SE responses are. $\endgroup$ – JYelton Mar 24 '15 at 15:13
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    $\begingroup$ @fredsbend I agreed it doesn't NECESSARILY scale up. We have fossil records of GIANT flying arthropods with wingspans over two feet: en.wikipedia.org/wiki/Meganeura and Arthropleura was 8 feet in length. So no, there can be plenty large insects even using earth physics and biology. Insect limitations have more to do with evolutionary pressures, structural stability and circulation than flight. $\endgroup$ – Isaac Kotlicky Mar 24 '15 at 18:24
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Can't answer based on aerodynamics or biology, but consider that most insects have four wings. This includes the dragonflies, which are amazingly adept aerial predators. So clearly, at the insect size level, two wings don't seem to have a significant advantage over four.

The real problem, I think, is one of developmental biology. ALL vertebrates have the same basic skeletal plan, including four limbs. This is based on the function of low-level HOX genes. (Search elsewhere for details.) So if your 4-winged birds are going to have legs as well, they pretty well have to come from a lineage that split off in the very distant past, likely coeval with Earth's Cambrian. And you're also going to have a whole history that has 6-limbed vertebrates evolving in parallel with the 4-limbed ones.

Another possibility is that the hind limbs have become modified into wings too, so the critters somewhat resembled pterosaurs: http://pterosaur.net/flight.php

Edit: Just thought of a really neat example of convergent evolution producing a pair of very similar flying creatures, one (hummingbirds) a two-winged vertebrate, the other (hawk moths) a four-winged insect. From casual observation, there does not seem to be any great difference in flying ability. See e.g. http://en.wikipedia.org/wiki/Hyles_lineata and http://en.wikipedia.org/wiki/Macroglossum_stellatarum Note though that despite the similarities, there are also major differences, the most obvious external ones being 6 legs vs 2, and scales vs feathers.

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  • $\begingroup$ That's one of the things that was wrong with the movie Avatar. Note that flys have two wings...the other two limbs are little counterweights not large surfaces, but a simple mutation could allow anoyher pair of wings to develop. Perhaps the regular wings have two limbs each, with the membrane stretched between them. Separating into two different wings would not require a chqnge in limb number. Flying squirrels evolve into four true wings, which are obviously better (even if the rear pair will later shrink away or further specialize). $\endgroup$ – JDługosz Mar 24 '15 at 6:52
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    $\begingroup$ This problem is contingent on Earth's evolutionary history and wouldn't apply on "a certain planet" unless there is a descent relationship between the vertebrates on Earth and the creatures on that planet. Another way to say it is that their lineage doesn't have to have split off in the very distant past if it was never connected in the first place. $\endgroup$ – Doug McClean Mar 24 '15 at 15:41
  • $\begingroup$ @JDługosz: With flies it's actually the other way around. The ancestral insect had four wings (as most still do), but in flies the rear pair evolved into the halteres. Flying squirrels might evolve four wings, as some pterosaurs sort of did: the problem is how they manage to get around on land while maintaining the OP's requirement that they look just like 2-winged birds. $\endgroup$ – jamesqf Mar 24 '15 at 17:48
  • $\begingroup$ @Doug McClean: Perhaps I didn't understand the original question. I thought the OP was setting up a planet with both four and six-limbed "birds", If the six-limbed ones are native to the planet, and the 4-limbed ones an import from Earth, of course evolutionary considerations don't apply. $\endgroup$ – jamesqf Mar 24 '15 at 17:51
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The four-winged falcons would probably have a net disadvantage. Certainly, it is likely that they'd be more manoeuvrable, however having two sets of wings would lead to lower propulsive efficiency due to the overhead of the extra appendages and the increased induced and parasite drag.

As a case in point, very few human-made aircraft these days are biplanes or triplanes, other than those designed for acrobatics, and certainly there are no commercial biplanes, as the extra rigging and wetted area increases drag dramatically. The only advantage in a multiplane design is to reduce the wingspan.

As for noise, the quad-winged falcons wouldn't even have that advantage. If the two pairs of wings flapped in synchrony, there would be no advantage, but if they flapped asynchronously or counter-synchronously (one pair going up while the other pair went down), there would be additional noise, like that produced by the interaction between a helicopter's main rotor and its tail rotor. Quiet flight (as in owls) is achieved by softer, more flexible feathers.

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  • $\begingroup$ I'm not sure I agree with this - the lift caused by fixed wing aircraft is drastically different than how birds generate their lift. A lot depends upon the configuration of the wings and their capabilities. $\endgroup$ – Isaac Kotlicky Mar 24 '15 at 1:04
  • $\begingroup$ @IsaacKotlicky, actually, its pretty similar. The difference in size changes the Reynolds number. Parasite and induced drag are going to occur regardless of size. Induced drag won't change much, but parasite will be higher. $\endgroup$ – Monty Wild Mar 24 '15 at 1:07
  • $\begingroup$ 2 of the 4 wings could retract close to the body when needed, so the additional drag is not a big issue, isn't it ? $\endgroup$ – Vincent Mar 24 '15 at 5:56
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    $\begingroup$ @Vincent - Retracting a pair of wings when not needed is not likely a viable option. It changes the center of lift from halfway between the wings to the center (approximately) of the unretracted pair. This will cause major attitude problems. $\endgroup$ – WhatRoughBeast Mar 24 '15 at 12:50
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    $\begingroup$ @JDługosz Biplanes were first made for structural reasons -- a biplanes wings form an extremely light and strong structure. This means a biplane can have much lower wing loading than an equivalent monoplane. If the bird's wings have to be self-supporting, then it loses the main advantage of a biplane. $\endgroup$ – cpast Mar 24 '15 at 22:46
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In nature, most things with four wings follow a rapid figure-8 rather than simply flapping two wings up and down for lift. A notable exception would be the hummingbird.

Unless the falcon completely changes the way its wings beat I doubt if it would perform any better than the two-winged variety.

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  • $\begingroup$ Plus, such a motion has a physical size restriction. There's a reason flying insects are only so big. $\endgroup$ – fredsbend Mar 24 '15 at 10:06
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The main benefit of having two pairs of wings is that the load per wing is reduced. Historically this allowed building heavier than air flying machines before wing construction techniques had matured.

For a bird using, presumably, similar wing construction as two winged birds, the benefit would be either shorter wing span or higher powered lift and agility.

Shorted wing span might be beneficial for a bird hunting in dense foliage. In real world this is handled with wider wings, but double wings might work just as well. The added agility from the extra control surface would also be consistent with hunting in dense foliage.

It is more difficult to think a benefit from extra lift since that would also require having extra muscle, I think the overall efficiency would go down. Maybe if there was a common prey that is just slightly too heavy for a bird this size to lift?

I think the agile predator in dense foliage getting an edge from shorter wing span and high agility is most likely scenario. But that is pretty narrow niche, so it is difficult to understand where the double winged birds would come from. Maybe deliberate genetic engineering?

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A four-winged bird might have different flight abilities, depending on how the wings are arranged and how it can use them effectively. It would tend to mean relatively more dedication of mass to wings, so a two-winged bird of the same mass would have more body and probably lower metabolism and less need for food.

A major survival advantage of having four wings could be if the bird can fly with one or even two (of the same couple) injured wings.

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What are the advantages of four-inch penis over two-inch penis?

It is not possible to answer this correct until nothing is said what is it about --- length or diameter?

While the detail (context) is unknown/unclear, prefer four instead of two: because having a more without a chance to potentially use is preferable over having a less with a chance to potentially fail :-)

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  • $\begingroup$ I'm asking about hunting/traveling abilities - or any of the other points addressed in the other answers. $\endgroup$ – HDE 226868 Mar 24 '15 at 20:12
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Four Wings a Disadvantage

I don't think four winged falcon-like creatures would be equal to the tasks of their two-winged brethren. This article from dailymail discusses a mutant bird born with four wings that is unable to fly as a result.

http://www.dailymail.co.uk/news/article-1278385/Hatch-day-Rare-chicken-guinea-fowl-hybrid-FOUR-wings.html

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  • $\begingroup$ 4-winged dragonflys are different than birds because of scale. The wings interact with whorls coming off the edges, with the width of the wing aboutnthe same size as the whorls. They have a very low mass and thus move quickly, so the air acts like a more viscus fluid would to us. $\endgroup$ – JDługosz Mar 24 '15 at 6:57
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    $\begingroup$ Chickens typically don't fly well already. It's also not a mutant, but more of a mule type thing (crossbreed); the article is wrong. $\endgroup$ – fredsbend Mar 24 '15 at 10:07
  • $\begingroup$ You are correct that the example given is a hybrid. But the article was not wrong. Any bird with additional limbs is, by definition, a mutant. The condition is called polymelia and it is a genetic mutation. In these cases, most extra limbs, when they occur, are neither functional nor useful. Therefore, a 4-winged bird merely has a set of little deformities that in no way enhance flight but would rather hinder it. petfinder.com/pet-news/… $\endgroup$ – DrAsimov Aug 4 '15 at 4:14
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It really all depends on which answer you want to hear. From a realistical point of view, a 4 wings falcon is a soon extinct specie, with no advantage at all over a 2 wings falcon.

OTOH, if you ignore the reality check and your focus is on "fantasy appealing", then your 2 wings falcon will be (as other answers pointed out) faster, capable of carryng heavier lifts, and much more agile.

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