From Aristotle to Leonardo Da Vinci, humans have been fascinated by bird flight throughout history. We made our first manned flight in 1783 and have since gone on to conquer the skies in increasingly advanced aircraft, many of which were inspired by the avian form. Birds, however, beat us to it by about 150 million years.
From the labored flapping but majestic soaring of the largest Eagles to the mind-numbing speed and agility of exquisite neotropical Hummingbirds, avians have truly mastered the mechanics of flight. Fine-tuned anatomy and the ability to harness lift, thrust, and natural air currents make bird flight a masterclass in aerodynamics and the laws of physics.
Here, we take a deeper look into the physics and mechanics behind bird flight.
As a means of locomotion, flight relies on both a vertical (lift) and a horizontal (thrust) force. Birds generate lift by flapping and the pitch of their wings, which are controlled voluntarily, but they also have some neat aerodynamic tricks up their sleeves.
Continue reading to learn about two important principles of physics that help birds defy gravity.
The upper surface of a bird’s wing is convex, while the underside is concave. This means air flowing over the top of the wing interacts with more surface area and must move at a higher speed. This simple characteristic allows birds to take advantage of an important aerodynamic concept.
Bernoulli’s Principle states that the speed of a fluid is inversely proportional to its pressure. Applied to birds, we see that the increased speed of airflow over the top of the wing creates a lower pressure than on the underside, creating a pressure differential. Since high-pressure air wants to equalize with low, it generates an upward current, providing birds with lift.
We’ve probably all heard of this one. This law states that every action or force has an opposite and equal reaction. So, a bird flapping its wings downward imparts a force against the air, and the air, in turn, imparts an upward force on the bird. The horizontal component of the wing and feather trajectory on the downstroke also results in forward momentum.
We’ve discussed the forward and upward forces generated in flight, but there are, of course, backward and downward forces that birds must overcome.
The most obvious hurdle to flight is weight (the effect of gravity), which imparts a downward force in opposition to lift. Birds have evolved ultra-lightweight bodies, and they defy gravity by applying force with their powerful wing muscles and utilizing the natural airfoil created by their wing shape.
Birds occupy space, so they must inevitably interact with particles of matter as they move through the air. These interactions cause friction and resistance, which result in a slowing force called drag. A bird’s streamlined wings and body shape are its primary defense against the effects of drag.
A Talamanca Hummingbird. Birds generate lift by flapping and the pitch of their wings, which are controlled voluntarily, but they also have some neat aerodynamic tricks up their sleeves
Flapping flight is an excellent example of Newton’s Third Law of Motion in Action. Birds push against the air below them by flapping their wings downward to generate lift. But if the downstroke of a bird’s wing creates lift, why doesn’t the upstroke push the bird back down?
Birds have control over the surface area size of their wings and the angle of their primary feathers in relation to the ground. This means they can decrease the air resistance of their wings on the upstroke and maximize the lift and thrust generated on the downstroke.
The difference in force generated is highlighted by the bird’s flight muscles. The muscles that power the downstroke are several times larger than those that power the downstroke!
Air is not static. The rotation of the Earth and temperature differences on its surface cause winds that provide ‘free’ thrust energy for birds. Lift is provided by the deflection of wind as it passes over slopes, but it also results from warm air rising off the earth’s surface.
Birds harness both of these forces with their specially shaped wings and feathers to generate lift and thrust. The obvious benefit of these flying techniques is that the birds capitalize on energy from an external source rather than from the metabolism of their food. The downside is that predictable air currents are usually restricted to specific topographies and regions of the globe.
Hovering is an advanced flight technique seen in relatively few bird species. While most birds impart thrust and lift forces greater than their weight and drag, hovering birds must balance these forces to remain in the same position relative to the ground.
This avian ‘juggling act’ allows birds like the Pied Kingfisher to hover motionless above the water before diving in and helps hovering Kestrels and Kites scan for prey on the ground below.
Hummingbirds, however, are the most skilled of the hovering avians. These specialized birds generate lift on both the upstroke and downstroke by flapping their wings in a figure-of-8 pattern.
A Pied Kingfisher. Hovering is an advanced flight technique seen in relatively few bird species
The real powerhouses behind bird flight are the antagonistic striated breast muscles that attach to the humerus of the upper wing.
The pectoralis majors are the largest flight muscles. When contracted, these powerful muscles pull the wings down, generating both lift and thrust. The smaller supracoracoideus muscles attach to the upper side of the humerus. These muscles pass over the coracoid bone and act like a pulley to lift the wings up for the next flap.
Flight is an energy-intensive form of locomotion, especially for birds that fly by constant flapping. It relies on advanced cardiovascular, respiratory, and muscular systems, so birds have evolved to efficiently supply oxygen and fatty acids to fuel their rapidly contracting and relaxing muscles.
Long-distance flights and migrations require a high metabolic rate and massive amounts of energy. Refueling along the way isn’t always sufficient, so migratory birds practice hyperphagia (overeating) to fatten up and store excess fuel for their grueling journey. Some species may even double their body weight in the weeks before beginning their migration.
Fat isn’t the only stored energy source for migratory birds. Studies on migratory songbirds and shorebirds have shown that some species deplete a significant amount of their protein and muscle mass during the journey.
Canada Geese in-flight. Flight is an energy-intensive form of locomotion, especially for birds that fly by constant flapping
Habitat plays a major role in flight mechanics, as the environment provides both opportunities and constraints to flight. Consequently, we see a wide variation in flight mechanics between birds of deep forests, semi-open habitats like urban areas, and the wide open skies above the land and seas.
For instance, long gliding wings would be useless to forest-dwelling songbirds since they must make frequent and rapid changes to their trajectory, and there just isn’t enough air movement beneath the canopy to provide the free energy required for this flight mode.
On the opposite end of the spectrum, short, elliptical wings that require constant flapping would be impractical for a heavy-bodied Albatross that must travel immense distances when foraging and returning to breeding colonies.
Migratory birds are capable of some extraordinary feats. Many species routinely travel at altitudes thousands of feet above the ground, and some have even been recorded at around thirty thousand feet!
Record-breaking species like the Arctic Tern may travel over fifty thousand miles every year, and species like Bar-tailed Godwit and Pacific Golden Plover can cover over five thousand miles without feeding or resting!
By appearance, migratory birds tend to have longer wings, with the point located toward the leading edge for more efficient flight. However, wing size varies greatly depending on other vital factors like the bird’s habitat and feeding strategies.
We’ve already touched on how birds store and use energy during migration, but there are some other important changes that occur in the bird body to facilitate long-distance travel.
Birds may change their diet to more energy-rich food sources, and some even partially absorb some of their organs to make space for the extra fuel!
High-flying birds survive in such low-oxygen (hypoxic) environments thanks to the extremely efficient avian respiratory system.
Unlike mammals, birds have a system of air sacs and lungs and even incorporate hollow bones into the mix. Their blood also contains a high number of red blood cells for efficient oxygen transportation.
Species like Bar-tailed Godwit can cover over five thousand miles without feeding or resting!
While seemingly simple on paper, overcoming weight and drag are remarkable evolutionary feats that have made birds the undisputed masters of the sky.
The secret to their success is a complex combination of specialized anatomy and physiology, working in concert with perfect timing and the laws of physics.
More than just a wonder to watch, studying bird flight offers key insights into their biology and ecology, which is important for the ongoing conservation of threatened species. We also owe many of our own advances in aviation to the beauty and perfection of the avian wing!
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