Ease of flight is deceptive
September 30, 2001

"Birds can fly, why can't I?" was the lament of a Yankee lad, in the poem "Darius Green and his flying Machine", written by John Townsend Trowbridge in 1910. It's not simple.

It's a matter of aerodynamics. Most everything about birds is aimed at meeting the two basic requirements for flight: low weight and high energy.

The obvious features for flight are wings and feathers. No single factor enables a bird to fly. They are virtually the same in all birds, regardless of size or species.

There are skeletal adaptions. The bones are thin and hollow. The bones are flexible. A strong keeled breastbone serves as an anchor for large flight muscles.

Weight is kept to a minimum. Even sweat glands have been eliminated. Birds pant to cool off. Reproductive organs are reduced in size, except during the mating season.

A bird's heart, although four chambered like a human's is proportionately larger...a humming birds is six times as large, and pumps at a rate of 615 beats per minute.

Like humans, birds breathe with their lungs, but they have additional air sacs which originate in the lungs and penetrate various parts of the body including the bones. This permits more available oxygen. One of the facets that allow birds to fly at higher altitudes than a bat, for instance.

The ability to fly allows for speed and maneuverability, assets in escaping predators and gleaning food or traveling. But a bird's nervous system and brain also have to react quickly as it may be required to fly swiftly through branches and leaves without disaster.

Speed calls for high energy and a bird's digestive system is efficient and fast, quickly converting high calorie foods into energy.

Birds in general have short lives, a toll that has something to do with flying. The high metabolic rate of birds is costly. It is to the bird's advantage to conserve energy by flying as little as possible. Birds seldom fly just for the fun of it.

It is the wing feathers and to some extent the tail feathers that provide the aerodynamic surface that makes flight possible for birds. All bird wings function on the same basic principle; the airfoil. On a bird's wing there is a greater curvature of the upper surface so the air has to go faster above than below, resulting in a pressure drop, or lift.

As with airplanes, the most critical times in flight seem to be taking off and landing. Birds with well-developed tails seem to be good at landing. A quick spreading of the tail causes a controllable stall. Getting airborne might depend on a breeze.

Always opposing speed and lift is drag. One way to minimize this is to have broader or longer wings, resulting in the variety of wing shapes in birds...that's another story.

Acknowledgements: this information was gleaned from the chapter entitled "Flight" from the Smithsonian Book of Birds, "Lords of the Air" by Jake Page and Eugene S, Morton