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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?
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.
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.
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.
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?
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.
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.
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.
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 :-)
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.
Maybe falcons with 4 wings fly using only one pair of wings during regular flight, in order to minimize drag. The other pair is retracted and can be considered as dead weight.
Yet it can use both pairs simultaneously, when it has to carry heavier payloads over greater range than the single winged falcon can.
Therefore it depends on the context for the 4 winged falcons to find their way in. For instance, small preys are becoming rare and those that are available are too heavy for the 2 winged falcon.
(Also some swallows are known to collaborate in order to carry coconuts. Single pair of wings falcon could learn from that)
