An unnecessarily drawn-out analysis of bird flight for animation
INTRODUCTION
In the course of evolution, we can find the development of flight in three major branches: birds, some mammals like bats, and insects. For the sake of this already quite long article we will only focus on avian flight. Bats' flapping flight can be roughly similar to birds', but insects' wings and patterns reside in a completely different category. Birds' behavior can change a lot from one species to the next. Since we cannot cover every species of bird, I've tried to keep the information here somewhat generic, with the idea that if you need a specific reference for your work you would be better prepared to investigate your specific niche independently.
I've divided the writing into three parts. We will start with understanding the anatomy of the wing that propels flight and the basics of aerodynamics that make flying a reality. From there we will be better able to understand, in part two, the different flight patterns and break down the three stages of flight: lift-off, flying, and landing. In the final part, I've covered some curiosities about wing shapes and how they influence flight patterns, why birds fly in formation, and take a separate look at hummingbirds specifically because of how particular they are. The purpose of this article is not to have a frame-by-frame breakdown but more a general understanding of how birds behave during flight. How certain species of birds have preferred flight patterns depending on the environment, wing shapes, size, and food source. My goal is to give you an intuitive feel for how birds will act and fly about their day.
Let's start with some good old anatomy first.
- PART I -
ANATOMY OF A WING
BONES AND MUSCLES
The realm of birds is vast and different and birds come in very different shapes and forms. Each species is adapted to its particular needs and lifestyles, so its anatomy can vary considerably from one to the next.
Often, when it comes to the anatomical structure of birds, most of its development revolves around the ability to be airborne. Generally, birds are trying to shave off as much weight as they can while putting most of their weight percentage into the muscles that are responsible for powering flight. Compared to human anatomy, their bones are partially hollow, which makes them lighter and with particular structural support that gives them added strength to resist the forces of lift-off, flight, and landing. Up to 25% of the bird's mass can be made up from the big pectoral muscle connecting the chest and the humerus. Contracting this muscle will pull the wing towards the front as in the downstroke. Opposite that, we find the supracoracoideus muscles, connecting the sternum and wrapping around the top part of the humerus. Contracting this muscle will rotate the wing upwards and pull it backward, powering the upstroke.
To avoid extra weight on the wing, unnecessary bones like the ones of the hand were either eliminated or fused together. While thick connections between bones and their fusion strengthen the whole structure, the distal part of the wing bone structure becomes rather inflexible - birds can't twist their elbow joints as we do. Bird wing bones got larger with fewer points of rotation.
To support the sturdy bone structure, a strong ligament connecting the shoulder joint to the wrist helps in preventing the wing from overstretching. Even in a fully extended position, the elbow joint always retains some bent.
The mobility birds lose through the structure of the wing they usually regain in the more proximal part of the body. Their shoulders are hypermobile compared to our shoulder joints (imagine being able to clap your hands behind your back while the arm is fully extended). Most of the capacity that birds have to change the shape of the wing and control dynamic flight comes from the folding and unfolding of the wing itself and the greater range of motion possible at the level of the shoulders. In short, the joints of the elbow and wrist can fold up but not rotate much, most of the rotation of the wing comes from the shoulder.
FEATHERS
Feathers have a wide variety of purposes in birds beyond the obvious one used for flight. Feathers help provide insulation from both strong winds and water, they help protect the bird from attacks by predators, and they are useful communication tools in form of shapes and colors for other birds and mating.
Because of their importance and delicacy, feathers require constant care and attention from their host to function well and be ready to fly. Most birds spend the majority of their time cleaning, oiling, and generally keeping their feathers polished and ready for action.
For the purpose of this article, feathers on the wing can be divided into 3 main groups: primaries, secondaries, and tertiaries.
Primary feathers attachments span from what we basically can consider the wrist to the end of the fingers, covering the distal layer of the bird's wing. The role of the primary feathers in flight is mainly to produce thrust (pushing the bird forward), and they are so essential that often just cutting the tip of the primary feathers is enough to ground the bird.
Secondary feathers are the ones attached to the radius bone (from our elbow to our wrist). They play a major role in creating lift (the upward force). Soaring birds have more pronounced secondary feathers to display as they use them to avoid flapping their wings too often to maintain altitude and try instead to make the most of the lift created by the wind.
Tertiary feathers are attached to the humerus (the upper arm) and they generally fill the gap in space between the secondary feathers and the body of the bird. They don't seem to play any major role in flight at the moment other than probably contributing to generating lift by filling up space.
Muriel Gottrop, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons (modified)
(Passeriformis)
(https://www.pexels.com/de-de/foto/brauner-und-grauer-kolibri-der-uber-orangenfrucht-schwebt-33101/)
(Trochilidae)
(https://pxhere.com/it/photo/1287345)
(Apodidae)
(https://commons.wikimedia.org/wiki/File:Falciot_3.jpg)
(examples of wing anatomy in different species)
Tail feathers are connected to fatty bulbs in the rear end of the bird. Contraction of the muscles changes the shapes of the tail feathers spreading them out like a fan. These feathers help the bird with more fine steering, braking, and maneuverability. Raising the tail helps the birds lift, lowering the tail feathers helps the bird break and it is often used during landing. Rotating the tail feathers left and right will help the bird change direction accordingly.
Due to their role in precise maneuvers, tail feathers evolved differently in different birds. They tend to be longer and more useful in birds who live in forests and need to navigate branches or tighter corners. They tend to be very short and almost disappear in birds that have little use for them like the ones living in open spaces such as big planes or the sea.
THE AERODYNAMICS OF FLIGHT
FORCES OF FLIGHT
At any point in time during the flight four forces are acting simultaneously on the bird: Thrust, lift, gravity, and drag.
Thrust is the force moving the bird forward, lift is the force pushing the bird upwards. These two forces are the ones the bird is actively trying to generate. Any moving object will encounter resistance to move (in this case mostly air, water has a higher resistance to movement due to its viscosity), and this resistance is drag. The balancing forces of drag and gravity act passively, slowing down the bird and pulling it downward respectively.
Needless to say, flying birds try to generate and maximize thrust and lift, while trying to reduce as much as possible the effects of gravity and drag. In birds, wings have the duty of generating both thrust and lift (unlike modern airplanes where the engine is a separate entity).
TWO THEORIES OF LIFT
Birds become and stay airborne by the way they manipulate the flow of air passing around their body, and around their wings more specifically. On a lucky day, this flow of air can come passively if strong enough winds are present (saving a lot of the bird's energy), or alternatively, it must be created by the bird itself through forward movement (thrust). Generating air flow is the reason why the creation of thrust is most important, thrust generates lift in the absence of strong winds.
We know of at least two ways that birds can convert thrust into lift.
Newton's law
The major lift-generating force comes from the angle at which the wing "hits" the upcoming flow of air, called the "angle of attack".
If we tilt the wing upward and run a strong gust of air at it, the airflow will push against the wall of feathers and be deviated downward. Newton's law says that for every action there is an equal and opposite reaction, in this case, this opposite reaction is pushing the wing (and the bird) upward and backward. The more air we can channel in this flow the greater the lift we can generate. Increasing the angle of attack of the wing will generate more lift, but generally only until around 15 degrees. Past this mark, the drag created by the steep angle of attack will be too strong to make flight energy efficient. This force is nullified if the angle of attack of the wing is parallel to the wind flow.
Note: you can have a feel for this force in action if you ever flew a kite in strong wind or by running. You can also experiment with this principle by putting a hand with fingers close to each other outside a fast-moving car. As long as the hand is parallel to the wind your hand will feel stable, but as you turn your hand upwards you can feel both the drag (push back) and lift (push up). Your hand is acting like a wing, even though it's a very inefficient one.
Bernoulli's principle
The shape of the wing itself also contributes to generating lift from thrust by manipulating airflow through the shape of the wing itself, known as an airfoil.
An airfoil is an asymmetrical shape of the wing that creates a difference in air pressure between the bottom of the wing and the top.
Due to its shape, the air on top must travel a larger distance in a similar amount of time. Because of this, air moves faster above the wing than it does below. This asymmetry creates a pressure difference with the faster-moving air having lower pressure. As things tend to move from areas of higher pressure to areas of lower pressure, this conveniently pushes the wing upwards (lift!).
Flying at low speed requires a lot of energy, so each bird is motivated to try and gain some velocity until it reaches the sweet spot for cruising. On the other side, aiming for too much speed can backfire as the amount of drag created by going too fast starts to increase exponentially past a certain point. It takes a lot of energy to go fast.
To be energy efficient, especially during long flights like migrations, every bird has a preferred cruising speed that sits right in the middle between too little and too much.
- PART II -
LIFT-OFF!
Most birds' main challenge of lift-off is generating enough thrust and momentum to begin ascending. Taking to the air requires a lot more energy and it is usually the most exhausting part of the flight cycle of lift-off, flight, and landing. To alleviate part of the energy costs of the lift-off, birds have learned to use different strategies and environments to their advantage. Below are five of the most common ways a bird is able to begin his flight:
DROPPING DOWN
If the bird finds itself in an elevated position such as a cliff or tree branch, dropping down can be the easiest way of gaining the speed and momentum it needs without putting in a lot of effort. Letting gravity do its work before regaining altitude with a few controlled flaps.
FLUSHING UPWARD
This lift-off has the bird giving a strong gush with it wings downwards to start getting elevation and momentum from static. This is probably the most expensive lift-off in terms of energy requirements and almost impossible to do for bigger birds. Smaller birds can be more at ease with this type of lift-off.
JUMPING
Birds with strong legs can help themselves get air momentum by using their legs for a strong push. The legs' power combined with a downstroke of the wing can be enough to gain elevation. This can be useful also for birds with a large wing span, as they won't have enough room for flapping their wings from the ground.
RUNNING
Large birds, birds with long legs, or birds with long wings can simply start running to generate enough forward momentum the same way airplanes do. This can happen in open fields or even on top of the water with a combination of flapping a running.
TAKE OFF INTO THE WIND
Because air impacting the wing is what causes lift, it doesn't make a difference if the bird runs fast or the wind is strong. The stronger the wind, the more lift can be produced by the bird without having to run to create it. Taking off into the wind can save a lot of energy. Contrary, taking off with the wind will require a lot more energy.
FLIGHT PATTERNS
Flapping its wings is only one of the ways a bird can fly in the air. Like anything else, a bird in flight always tries to save energy and make the most out of the energy he finds in the environment. The landscape, the shape of the wing, the diet of the bird, and the climate all together often dictate how, when, and where a bird flies. This can lead to one of these most common forms of flight:
UNPOWERED FLIGHT
Unpowered flight isn't really unpowered as much as it is the bird relying mostly on external forces for movement (wind currents or gravity).
GLIDING
Gliding is nothing more than a slow descent and is probably the most evolutionary basic form of "flight". In this case, a flight is powered by gravity, and the bird is resisting the force of gravity and creating forward momentum. Gliding only works as long as the bird is moving toward the ground, if it tries to ascend it will quickly lose speed and get to a stall.
SOARING
Soaring is a more sophisticated version of gliding where the bird is still technically descending relative to the air but has learned to make use of the current to gain actual elevation without the need to flap its wings. Most of what a soaring bird does is to search for rising currents that can often come from rising heat from the ground, the wind blowing on the slopes of mountains, or on the open ocean. Because of the strong dependence on the air currents, soaring is often limited to specific environments.
POWERED FLIGHT
FLAPPING FLIGHT
Flapping is the basic form of powered flight for any bird and the cycle is divided into two main movements.
The downstroke moves the fully extended wing from the top and backward position to the front and downward position. In the middle part of the downstroke, the leading edge of the wing is tilted downward, the feathers are pushed together creating a wall for the air to push against, and the tip of the primary feathers tend to be bent backward due to the air pressure. The downstroke is the most important stage of the flapping wing as it is the one creating thrust and lift. The downstroke often lasts slightly longer than the upstroke.
The purpose of the upstroke is to return the wing to the up and backward position needed for the downstroke without disrupting the momentum gained and creating the least resistance possible. The wing starts from the extended position of the end of the downstroke. From there, the leading edge of the wing tilts backward while the wing folds up to minimize air surface and with it the drag created. The feathers during the upstroke spread up to let the air flow through them without much resistance.
Wings' flaps can look very different depending on the bird, its volume, and in the amplitude of movement (a soaring bird can use only small half flaps).
HOVERING
Hovering is maybe the most exhausting type of sustained flight. The bird is standing still, losing all lift generated by thrust that needs to be compensated with strong and fast wing flaps. A small hovering can sometimes be used before landing from all birds. Prolonged hovering is mostly used by small birds whose body weight and dynamic wings make this feat a lot easier, or it can sometimes be used by bigger birds as a momentary stance to stabilize and track their prey on the ground before diving.
BOUNDING FLIGHT
A bounding flight is characterized by short bursts of fast wing flaps alternated with short pauses where the wings are drawn in. It is often used by small birds that are able to flap their wings fast and it is probably more effective in navigating trees in the forests. Bounding flight creates a recognizable oscillatory movement up and down of the bird as it gains and loses elevation quickly. This type of flight can help the bird save energy.
Larger birds might use a variation of bounding flight where they glide for some time with their wings spread before using some powered flapping to regain the altitude lost.
LANDINGS
Lift-off revolves around power generation and being able to create enough energy to transform thrust into lift. Landings are more about coordination, timing, and grace. Landing is a fine balance between the timing of speed and gravity where it needs to come to a low speed and lower its altitude delicately at the same time. The main problem in landing is being able to lose the thrust needed during the flight at the right time compared to the ground position. If the bird loses too much forward momentum too high in the sky it will just fall down. If the bird doesn’t lose enough forward momentum when landing it will basically crash onto the ground.
To lower its cruising speed, a landing bird often increases the angle of attack of its wings and flaps them forward.
This increased angle of attack creates a characteristic "agitation" of the flow of air behind the wind, ruffling visibly the feathers in their back. If possible, a bird will always prefer to land against the wind, it makes it easier to hover at slow speeds without losing lift.
Bigger birds or birds that have quite a large wing span have usually a more difficult time landing gracefully than smaller ones because they can't flush their wings as fast to come to a halt. Birds with large wings and small legs like albatrosses have a much harder time landing because as they get too close to the ground they can't fully flap their wings due to the lack of available space.
This is Jerry, and he gives an example of losing too much altitude and not slowing down enough. Landing can be very difficult, ungraceful and doesn't always go well.
As with lift-offs, birds have strategies that can help them land gracefully in the right setting. These are some of the most common landings:
PULL UP LANDING
Used for birds that are landing on tree branches or a spot that is in an elevated position like a cliff. The bird starts the landing by aiming lower than its target and pulls itself up at the last moment. The force of gravity will slow down its speed and forward momentum.
This landing takes a bit of mastery on the bird's side but can be very effective in saving a lot of energy, granted that there is room for maneuvering in the space below the landing zone:
HOVERING LANDING
It is probably the more expensive in terms of energy. The bird flaps its wings stronger and with a greater angle of attack until it finds itself in a hovering position close to the ground from which it can simply let itself drop safely:
RUNNING LANDING
It is the most similar to an airplane landing, the bird stretches out its feet and touches the ground with still a lot of forward momentum. It slowly morphs the flight into a run on the floor and tries to slow down after contact.
This landing is used mostly by bigger birds that often also happen to have long legs to make use of:
WATER LANDING
Very similar to the above, but in this case, the landing surface is much softer. The bird can slow down its forward momentum by slowly touching water and creating friction.
Where water is available, generally the bird will prefer landing on water rather than performing a running landing:
- PART III -
WING SHAPES, FLIGHT FORMATIONS, and more
WING SHAPES
Understanding wing shapes is important because they can tell you a lot about the bird and its flight patterns just by looking at it. With a glance, you can understand how this particular bird usually flies, how much energy it spends in flying, how long can it fly, how it likes to land and lift off, and potentially even the environment it lives in.
To understand wing shapes there are three main factors we need to consider, the first one is wing-loading. Wing-loading defines the relationship between the weight of the bird and the surface area of the wing. Low wing load means the bird has a small body and big wings, high wing load means a big body with relatively small wings.
The lower the wing load, the easier it is for the bird to drift and make use of the wind currents.
Birds with low wing loading generally have a harder time flapping their wings fast and deeply, but they are very efficient passive flyers. They need a lot of extra speed for taking off, which is often translated into a running lift-off. Birds with high wing loading on the other hand can often lift off from a static position or a jump, but they are not as energy efficient at flying long distances.
The aspect ratio defines the relationship between the length and the width of the wing. The higher the aspect ratio, the thinner the wing is. The lower it is, the more squared the shape of the wing will be. Higher aspect ratio wings create more lift when flapped, can make better use of wind currents, and make the flight significantly more stable, but are not as dynamic as the short ones. What short and broad wings offer is greater maneuverability, they can be flapped faster and with higher frequency. Naturally, long and thin wing birds are often found in open spaces while short and broad are found in forests.
For reference, this is the same difference between a commercial airplane with long wings designed for flying stable in a straight line from A to B and the shorter wing design of a fighting jet created for high maneuverability.
There is some evidence suggesting that the shape of the tip of the wing itself can change performance during flight. Pointy tips seem to help reduce drag and be most common for birds that are concerned with speed such as falcons, or for migratory birds like ducks that have to travel long distances (the same concerns are not there for soaring birds that just need to get more airtime rather than travel far).
Rounded tips on the other hand can be more effective in lift-off by increasing the angle of the lift-off and maneuverability that can come into play when escaping predators.
FLIGHT FORMATIONS
Birds that travel long distances will prefer to fly in formation. V formation helps them save energy by utilizing the upwash of air created by the wing of the bird in front.
The shape of the wing of a bird redirects the flow of air in such a way as to create a whirlwind at the tip of it. Air is being pushed down and then up again in a circular motion. If the bird that is flying behind is positioned in the right spot at the side of the wing it can make use of the rising air to generate lift.
The leader of the formation is obviously the only one not benefiting from the upwash and is the one getting tired faster, but the leader often gets replaced as each bird gets some rest in the back before contributing by leading the group.
SOME FUN FACTS ON TERRESTRIAL MOVEMENT
If there is a chance, a bird will always prefer running or hopping to move rather than flying, it requires a lot less energy.
Some birds hop and others walk or run. Usually, hopping birds are tree born and they are used to hopping as a safer way of navigating branches. Birds that live on the ground or water like ducks prefer instead to walk or run.
It might look like some birds have knees pointing backward, but that is not the case. What we mistake for a knee is in reality what in humans corresponds to a heel.
HUMMINGBIRDS
Hummingbirds deserve their own small chapter for how particularly they developed. They contain almost all the exceptions I've mentioned in this article in one species.
Hummingbirds are small, highly specialized birds with pointy and very dynamic wings that are known for the impressive high frequency with which they flap their wings, which can vary from 12 to more than 80 a second, depending on the bird. They are one of the few species that are very comfortable with hovering (necessary for feeding on nectar from flowers) and the only known species that can actually fly backward. Flying for a hummingbird is highly expensive but it can handle the workload by having very energy saving hybernations during rest and by surviving on a diet mostly based on nectar (high in glucose and full of easily usable energy).
Their flight pattern is also quite different from the rest of the bird kingdom, more similar to the dynamics of a helicopter rather than an airplane.
The shoulder joint is hyper-flexible and allows the hummingbird to actually have two downstrokes per wing flap cycle - the upstroke is turned into said second downstroke. With the classic downstroke the wing goes down and front, pushing the body slightly backward; in the second downstroke the bird raises the wing up and frontal, then lowers it down and backward during the stroke, with the wing flipped upside-down. This pushes the body of the bird slightly forward again and helps to compensate and keep the hummingbird stable in the air. On top of that, the head compensates the body to maintain very still, especially while feeding.
The specific way they flap their wings creates the recognizable 8 pattern on each flapping cycle (which is invisible to the naked eye due to the high frequency of flaps)
Fun fact, the small lower limbs of the hummingbird are very limited and seem only used for perching on branches and making very small steps sideways on them. Hummingbirds do not hop or walk. No matter how short the distance, if they want to move they will simply lift off and land in the new position.
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Bibliography and references
Alexander, David E. On The Wing.
Alexander, R. Mcneill. Exploring Biomechanics.
Burton, Robert. Bird flight: An Illustrated Study of Birds' Aerial Mastery.
Denny, Mark, Mcfadzean, Alan. Engineering Animals.
Henderson, Carrol L. Birds in Flight: The Art and Science of How Birds Fly.
Laws, John Muir. The Laws Guide to Drawing Birds.
Kaiser, Gary W. The Inner Bird: Anatomy and Evolution.
Muybridge, Eadweard. Animals in motion.
Taylor, Marianne. The Pocket Book of Bird Anatomy.
Bird basics: Six different feather types explained:
Bird wing:
https://en.wikipedia.org/wiki/Bird_wing
Inside a wing:
https://www.dkfindout.com/us/animals-and-nature/birds/inside-wing/
Bird Wing Anatomy with a Diagram – Bones, Muscles, Joints, and Vessels:
https://anatomylearner.com/bird-wing-anatomy/
BIRD FLIGHT:
http://vireo.ansp.org/bird_academy/bird_flight.html
Bird flight:
https://en.wikipedia.org/wiki/Bird_flight
Deconstructed Bird and Insect Wing Patterns by Eleanor Lutz:
https://www.thisiscolossal.com/2014/10/deconstructed-bird-and-insect-wing-patterns-by-eleanor-lutz/
V formation:
https://en.wikipedia.org/wiki/V_formation
Hummingbird:
https://en.wikipedia.org/wiki/Hummingbird
Bird Anatomy:
https://en.wikipedia.org/wiki/Bird_anatomy
Bird Flight:
http://people.eku.edu/ritchisong/554notes2.html
Pictures:
Flying Yellow Bird
https://www.pexels.com/photo/flying-yellow-bird-459198/
Close-up Photo of Flock of Flying Seagulls
https://www.pexels.com/photo/close-up-photo-of-flock-of-flying-seagulls-110320/
Black Cormorant Flying Low over Blue Ocean
https://www.pexels.com/photo/black-cormorant-flying-low-over-blue-ocean-11745122/
A Bird Flying Near Water Surface
https://www.pexels.com/photo/a-bird-flying-near-water-surface-10625663/
Selective Focus Photography of Hummingbird
https://www.pexels.com/photo/selective-focus-photography-of-hummingbird-469315/
Close-up Photography of Green and Brown Bird Flying Over Body of Water With Catch On Its Beak
Videos used for the samples:
Relaxing Super Slow Motion Birds: https://youtu.be/HcNgYI9H0BM
Linnet - Birds On and Off The Branch: https://youtu.be/RXDeOMy9ML8
Blue tit,s in slow motion: https://youtu.be/QKTrJm0hZrE
The Anatomy of Flight: https://youtu.be/YhWbKvR_GBA
Birds Take Flight - video reference for animators: https://youtu.be/CJHP6dPjuGY
Flying swan. Take off and landing. - Slow motion: https://youtu.be/pcnUPJoh15c
albatross takeoff: https://youtu.be/lwhmNC4yhvU
Flamingo flight movie: https://youtu.be/NEby2vZ0i4k
Albatrosses Use Their Nostrils To Fly | Nature's Biggest Beasts | BBC Earth: https://youtu.be/SRTRRMwXuEg
Gliding birds: https://youtu.be/mbVqiH3GR7I
BIRDS IN FLIGHT: https://youtu.be/a1wp1RnC7kk
Slow motion birds in flight: https://youtu.be/Lpog3yi6jr0
Bird hovering in slow motion: https://youtu.be/5rMSdeh5x-w
19 Bounding flight: https://vimeo.com/187333450
Bald Eagle Slow Motion Flying Display & Close Up - Birds of Prey: https://youtu.be/WoQM1fGs5xE
Albatrosse starten: https://youtu.be/mN3zz6PcC0w
Linnet - Birds On and Off The Branch: https://youtu.be/RXDeOMy9ML8
Birds gliding through bubbles reveal aerodynamic trick: https://youtu.be/2sh8_3-R90I
How Fast Can a Hummingbird Flap?: https://youtu.be/naE7fK-gCPs
Slow-Mo Barn Owl in Flight | Unexpected Wilderness | BBC Earth: https://youtu.be/hlKo42iPslg
https://www.photojoiner.net/ for the photo edit
https://www.draw.io/ for the diagram
Efficiency is key in a healthy walk cycle, with propulsion and energy conservation being the primary objectives.
In this article, We are going to break it down into its most basic components. First, we will break it down temporarily into small chunks and actions. Second, we will break it down physically to understand all the components and movement patterns that go into a step.
But before we do all of that, we will look at the gait cycle from afar, cover some general knowledge about human walking, and understand the few forces that are actually pushing our body forward.