The Aerodynamic Origin of Bird Flight

Why, How and Where Bird Flight Evolved


The Aerodynamic Origin of Bird Flight

How, Why and Where Bird Flight Evolved


This theory is an aerodynamic approach, how natural selection evolved powered flight and the features of birds. It will explain how flight evolved, why it evolved and where it evolved. It will explain the contiguous incremental successes that continuously benefited the survival of "protobird". Contiguous is the "how" natural selection works; related changes to existing anatomical features. Continuous is the "why" natural selection works; the benefit for survival is constantly available, or at least seasonally, to promote evolution. This theory will explain the initial aerodynamic act and the subsequent changes.


About 150 to 200 million years ago, a small bipedal dinosaur (theropod) did something new. It did something that used an anatomical feature in a new and aerodynamic way. What was this act and why was it successful? How did natural selection preserve this favored race? How did it evolve into modern bird flight? And, is it possible that some modern birds still do something like it today?


Evolution and natural selection are driven three ways: offensive, defensive or reproductive. An incremental success in one of these areas will be favored and will be carried on by its offspring. I do not believe that reproduction or mating drove anything aerodynamic. Protobird could have had extended thermal protection feathers for display or to cover their young from rain or the sun but these feathers would not succeed aerodynamically as I will explain later. Tree nesting, migration, etc, would come later. Defensive jumping can lead to parachuting or paragliding, but if the escape is successful further changes would cease. I believe that a new offensive technique to reach an unexploited food source drew protobird forward and created a new species that became birds.


The two main theories for the evolution of bird flight are the "ground-up" and the "tree-down" and variants. These theories are deficient because they fail to explain the first contiguous incremental aerodynamic success that would continuously benefit survival.

Ground-up theories usually have the feathered forelimbs extended out for lift or deployed forward for a thrust stroke or up for a lift stroke. The why, is that they pursuing low flying insects or fleeing a predator. But, in any case, the drag would initially exceed the lift or thrust. It is physically impossible for this to evolve! The success can only come if the lift to drag or trust to drag ratio is greater than one. This is not the case. A track sprinter takes off his warm-up suit to reduce air drag. A four legged adaptation and low drag shape would be far more successful, but its forelimbs would be legs not wings.

The tree-down theories have two problems. Bi-pedal animals don’t climb trees. Why and how could a theropod climb a tree? If it used its forelimbs to do so and their survival benefited, then grasping and the claw would evolve to do this better. And the why, (continuous benefit to survival), is not explained. Protobird didn't climb trees to practice flying. And secondly, if protobird was already a tree dweller, what would they accomplish jumping limb to limb, tree to tree or tree to ground that they couldn’t do by climbing? These scenarios are inherently dangerous and evolution does not favor risky behavior unless failure is benign. Arboreal animals jump with their hind limbs and grasp with their forelimbs like monkeys and squirrels. If they jumped and climbed they would need their forelimbs to do so. Here again, a four legged adaptation is favored.

Defensive jumpers will never flap, and how I came to this theory.

Some squirrels, to escape a predator, would have been forced to drop or jump to lower branches or interleaved branches from other trees. These squirrels would adapt to limit their speed so they could grasp lower branches or land on the ground. Limiting speed with drag is called parachuting. Flapping is used by birds to increase speed and there is no explanation (continues benefit to survival) why a parachuter would try to speed up while they are trying to slow down. What would be the incremental success that could be rewarded by natural selection? There is no explanation, and this led me to conclude that protobird was not a parachuter and wanted to go fast! It then occurred to me what the initial act was, and how it was successful. I also realized that some modern birds perform it to this very day!

Another defensive jumper, worth discussing, that will never flap is the flying fish. The flying fish jumps out of the water to escape a predator. It glides along supporting its entire weight on its enlarged pectoral fins. When it slows down it drops its lower tail in the water and wags it fast to accelerate. Their weight is supported on their wings not with muscles but with bone structure. They do not have the muscles to pull their wings down, only to deploy them. And again, there is no explanation of what some incremental success would be if they could begin to pull their wings down.


If we observe modern birds or airplanes today we see three kinds of aerodynamic forces in use. The main wing uses the classic airfoil shape to produce maximum lift with minimum drag. This force is usually referred to as Bernoulli (Daniel) lift. The tail, which is a flat or thin symmetrical fin, is an air deflector and is one of the forces known as dynamic lift. The third force, I will not discuss at this time. The third force will be the clincher of my theory because it will explain where flight evolved!


To evolve a small anatomical feature into an aerodynamic appendage will take much more air speed to achieve some success, than later to use it. Hold your arm out a car window, flatten your palm, at say 30 mph and deflect the air down. Your arm doesn't lift and since there is no success at anything there can be no evolutionary change. Now increase your speed until your arm lifts. Here we have a small incremental success and if survival benefited, natural selection would favor those with the larger palms or lighter arms. Eventually lift at lower speeds will evolve. Lift at 30 mph will come from higher speeds not from below. A micro wing can only be useful at macro airspeed.

Another example; a board diver will land in the water in a predetermined place because there will be not enough force generated to change direction. But a ski jumper uses his arms and hands to stabilize and control his attitude. Small wings or fins will be useful only if there is major airspeed to achieve any kind of incremental success.


Take a hand thrown balsa wood glider and remove the wing. When thrown, it will go straight like an arrow or dart. But if you only remove the tail pieces, it will go completely out of control. It seems reasonable that stability, guidance and attitude control would precede lifting, turning or banking. This is my second conclusion about protobird, its tail evolved first, so it could stay straight!

An arrow cannot turn. Even if you could articulate the feathers it might yaw or pitch but it can't change course because it has nothing to turn against, it needs a forward wing or fin. As a jet plane thrusts down the runway it reaches a certain speed so as to rotate the nose up. The leverage is against the landing gear. During flight the leverage is against the wing. The evolution of the wing can only come if the tail can control its orientation. The tail has to come first!

So protobird wanted to go straight and fast, but what was it doing? What was the initial incremental aerodynamic success that allowed protobird to evolve to do this better?

Combine It

There is one other conclusion that I made from protobird not being a parachuter. What is the observable difference between parachuting and gliding? The answer is that in parachuting the feet must go first. The feet must be ready to grasp something or land on the ground. But with gliding, the head goes first so you are streamlined and fast.

What was the initial act? What is it that some modern birds do to this very day, head first, straight and fast without the need of a lifting wing?

The Bank Jumping Plunge Diving Theory (BJPD)

One day, about 150 to 200 million years ago, a small bipedal dinosaur (theropod) did something new. It dove from a riverbank or lake overhang to take a fish, head first. It swam or walked back to shore. If it missed, it walked and climbed back up to the spot from where it dove, and waited for another fish to come by. Water is the key, because almost any animal can dive in without hurting themselves. Since only birds take marine and aquatic food from above, that this was the new and unexploited food source that drew protobird forward.

Offensive failure by diving is not harmful. It would be tried and tried again from rocks, banks and cliffs, those that succeeded would benefit and through natural selection create a new species that would start the evolution to modern birds. Offensive trial and error, to find new food sources, is instinctive and is a common and basic trait to all animals.


Protobirds were hunters and waders. Protobird would be semi-aquatic and would have no fear of water. They would take food with their mouth or run and jump to take fish in shallow water with their feet. Wading is a very successful method of feeding, and is used to this day by countless birds. Hunting feet first would use the drag of their feathered forelimbs to quickly bring their feet forward.

Protobird could swim. They would venture into ponds, lakes and swamps for food. They would have to get around. It is not necessary to have fins or webbed feet to swim just enough buoyancy and a dog-paddle action to move. It is also possible that the use of the forelimbs for swimming would help them later to flap.

Protobird was warm blooded and feathered. These come together because warm blooded animals need to keep warm throughout the day and night. They also need something to help keep their eggs warm and shade them from the sun. The early protobird gets the worm. Warm blooded protobird would attack at dawn, as soon as it could see. The pterosaurs would have to warm up. Buoyancy is another use for feathers. Feathers would help them to float and swim even if temporally.


One thing that has never changed are rivers. As a river flows down a plain it meanders. The inner arc with slower current deposits silt and grows. The outer arc is a cut bank providing the silt for depositing later down stream. Meanders grow until they are shunted by floods leaving islands, coves and sandbars. This is also why the fossils are so rare. This area would be eroded away within a few generations, let alone thousands. There are now and there would have been many places where you can see fish from the shore, banks and cliffs. Protobird would expand its wading area by adding the edges and the areas that were normally bypassed where the water was too deep.

Contiguous Incremental Successful Aerodynamic Changes

So if there are fish to dive for, protobird will try. Attempts will be made from various terrain and heights. Higher up will mean more speed which will allow some small anatomical feature to be used aerodynamically. It will most useful to stay straight, head first. A passive trailing arrow tail will suffice. If the body stays straight, jumping errors can be minimized. The neck and body can be used to aim the mouth. The swim, walk and climb back maybe long so accuracy will be important. Those that did it best would survive and separate from the waders.

The tail is the first exaptation. Early rockets had a stick tied to them to keep their flight path more consistent for aiming purposes. The function of the tail will change from a balance control to aerodynamic steering. A headfirst diver will need to control its roll, pitch and yaw to keep its aim straight. Protobird will begin to use the drag of their forelimb feathers for yaw and the tail for pitch.

Incipient flight feathers would help both feet first and head first attacks. A forelimb fin will allow protobird to pitch up or down and roll left or right. In aircraft it's called turn and bank. Pitch will allow protobird to jump outward to clear non-vertical situations. Forward canard steering is common in guided missiles, which is what protobird is.

However, it's more efficient for the tail to control the pitching. The tail must become active and responsive. Less deflection and drag will come if the control is from the rear. Protobird will use its feathered fingers to control roll. On aircraft the elevator is in the rear and the ailerons are on the outer wings, far from the center of mass.

Flapping will begin to increase the effectiveness of the forelimb fin. Flapping will actually precede level gliding. Protobird is still in a dive and cannot yet generate enough force to raise its glide slope much. A 1 to 1 glide slope ratio is a 45 degree angle. Modern gliders have glide slope ratios in excess of 40 to 1.

As the forelimb fin pitch increases and so will the glide slope. The fin/wing will need to be longer, larger and more efficient. Asymmetrical feathers will shape the airfoil and Bernoulli lift will increase the lift to drag ratio.

Overall maneuvering will increase. Protobird will succeed in taking flying insects over the water. Protobird will learn to maintain control when bringing its feet forward to take larger fish too big to swallow whole. Protobird will swoop into ponds and marshes to take amphibians, insects, snails and worms.

More Lift

Protobird stands on a cliff top overlooking the river. It sees something across the river. It's a long flight; it may have to belly-in, then swim and run to catch it. It jumps off, forward toward the prey. It adjusts its direction to compensate for the wind. It settles into constant glide slope and speed. It pumps its wings to raise its glide slope a little. It’s getting near the water. But just before it touches water it feels an increase in lift. It's gliding level now, for the first time, riding an air cushion. It's riding on the third force!

As a wing at speed comes within its chord length to a flat surface a pressure wave compression occurs beneath the wing. This allows more lift to be created at lower speeds than clear air Bernoulli lift. This can be routinely observed by watching pelicans glide along over flat water without flapping. This is why the third lift clinches my theory. The ground is too uneven and has obstacles. Protobird must have gained flight over flat water. This is where flight evolved.

Up and Away

Eventually after millions of years protobird, when the wind is right, will lift above the cliff ridge. From here it will see new horizons. As the new species diverge some will pursue new opportunities away from the river. Some will explore up or down the river valley. Some will fly away and never return. Some will seek new places to have and protect their young. But they will succeed, of course, because birds are here today.

Copyright © 2007, Arthur H. Tarver, All Rights Reserved
Ver 4, Dec 2008


I've had this posted a year now. Most criticsm has been semantic and not about BJPD. Yes it's crude, amateurish and without citations. It's an essay to turn a hypothesis into a theory.

I was on small pier looking down on school of a thousand anchovies. I waved my arms and there was no reaction. But, when the shadow of my arms hit them there was an immediate reaction. Fish cannot see you coming from above!