Sometimes we are so busy looking at the differences between birds that we forget the similarities.
Birds resemble each other in many ways. Perhaps none is as obvious as shape. This shouldn’t be surprising, since shape is critical to aerodynamics, and birds, from their first appearance in the fossil record, have shown adaptations for flight.
The bodies of all flying birds are shaped like teardrops. The streamlining is achieved by specially arranged feathers that reduce the friction that would otherwise act as a drag against the forward-moving body.
Look at a plucked bird the next time you get a chance. Its huge breast muscles would offer large blunt surfaces to onrushing air. When the feathers of the lower neck are in place, however, the same structure appears smooth and streamlined. And the same is true at the rear of the bird, where the tail feathers and feather groups above and below the tail form the smooth, tapered end of the teardrop.
Think about all of the flying birds you know. Their shapes are remarkably similar, even though modifications for different lifestyles cause variations. It’s not hard to come up with examples: long necks and legs for feeding, long tails to serve as rudders, flat faces to increase binocular vision, and wings that are broad, or long and narrow for soaring, pointed for speed, and short for diving.
Even birds that vary greatly in size share the body shape. The birds in the accompanying illustration range in size from Cuba’s Bee Hummingbird, the smallest flying bird, to North America’s Trumpeter Swan, one of the largest.
The hummingbird is only about two and a quarter inches long, and it weighs about eight-hundredths of an ounce. The swan is 60 inches long and weighs about 30 pounds. Trumpeter Swan, therefore, is heavier than Bee Hummingbird
by a factor of 6,000. But the swan’s 96-inch wingspan is only about 24 times longer than the little bird’s four-inch spread.
Smaller birds have relatively longer wings than larger birds, presumably to ensure adequate surface area to maintain lift and flight. If the hummingbird, for example, had a wingspan in proportion to its weight similar to the wingspan-weight ratio of the swan, the hummingbird’s wings would be less than half an inch long — too short to fly.
The aerodynamic teardrop shape that adds to the efficiency of flight proves to be of similar value to swimming and diving birds. Penguins, for example, spend most of their time in water, where they fly through the medium using their small flipper-like wings. In a similar way, auk-like birds swim and dive, propelling themselves with wings, feet, or both. Again, this isn’t surprising, since water is also a fluid medium, albeit heavier than air.
The cross-section of a wing has a teardrop shape as well. It’s thicker near the leading edge and tapers toward the trailing edge, and the upper surface is curved more than the lower surface. Such a structure is called an airfoil. When passed through the air, it generates lift.
For years, the most popular explanation for lift was based on the notion that air hitting the front edge of the wing splits. Some passes over the top and some goes underneath, and all of it arrives at the trailing edge at the same time. (This is known as the equal-time hypothesis.) According to conventional wisdom, because the upper surface is longer than the lower surface, air flowing over the top has to travel faster than the air underneath. Consequently, less pressure is generated on the top of the wing than on the bottom (Bernoulli’s Principle), and the pressure difference causes lift.
But the equal-time hypothesis just ain’t so — that is, the airstreams do not arrive at the trailing edge at the same time. So how is lift created?
When the air moving over the top of the wing passes over the trailing edge, it is deflected downward (downwash), below horizontal. According to Newton’s Third Law of Motion, the downwash causes an equal and opposite force on the wing, creating lift and thrust.
In fact, both Bernoulli’s Principle and Newton’s laws contribute to lift. The greatest amount of lift is created when air striking the leading edge of the wing is deflected upward, pulling air from above the wing with it. The action leaves a large volume of reduced pressure on the top of the wing, which generates a large amount of lift. The value of a teardrop-shaped wing is that it creates lift at a smaller angle of attack (that is, the angle between the wing and horizontal) than a wing shape that is flatter.
As diverse as the birds of the world are, the similarities in structure and function are profound. Certainly, one of the most prominent is the teardrop shape. It unites all of the flying and swimming birds into the amazing avian assemblage we enjoy today.
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. This article appeared in the April 2017 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founder of Birder’s World magazine.
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