It's a Bird, It's a Plane, It's a...a...Fish!

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Fish aren't birds. Seems like a simple enough argument; one lives in the water, the other flies around and lives pretty much wherever it likes.

Then there are flying fish. Like flying squirrels and scuba divers, these animals appear profoundly confused about which element they belong in. They blur the lines about what it means to be a "fish." The thing is, they're good at it — flying fish can remain aloft for up to 45 seconds and travel a quarter of a mile above the water. 

WATCH VIDEO: Flying fish take off in this incredible video

How do they do it? Two engineers at Seoul National University in Korea, Haecheon Choi and Hyungmin Park, have just found out.

Previous studies determined that these fish initially swim extremely fast and gain momentum before bursting forth from the water. Once airborne, they glide with their wing-like fins spread out wide, looking very much like a bird or insect. When they need an additional speed boost part-way through their flight, they beat their tails against the water in a motion called taxiing.

But, until now, researchers had no idea just what efficient fliers they were. 

There are about 40 known species of flying fish. They are all equipped with two large pectoral fins that serve as "wings" in the air. Some also come with additional back or pelvic fins and are called "four-wingers." 

Particularly interested in uncovering the mechanisms of four-winger flight, Choi and Park analyzed darkedged-wing flying fish (Cypselurus hiraii) from the East Sea of Korea. The fish were stuffed in a way that maintained their body size and wing flexibility, sensors were attached to different points on their bodies and they were mounted in a wind tunnel. 

Researchers tested the lift-to-drag ratio of the fish — that is, the measure of horizontal distance covered relative to change in height during a flight. They observed the animals' aerodynamics when tilted to different angles and when lifted above both a solid and liquid surface. 

In a recent article in the Journal of Experimental Biology, Choi and Park reported that when fish fly close and parallel to the sea surface they reduce their drag, increasing their lift-to-drag ratio, and thus maximizing flight efficiency. 

In fact, according to the researchers their gliding ability "is comparable to those of bird wings such as the hawk, petrel and wood duck."

Though Choi and Park have solved the mystery of how these fish fly, scientists are still unsure why they fly. The leading theory is that it enables them to avoid predators. Another idea is that the combination of flying and swimming is a more efficient mode of transportation that saves the fish energy.