From: Kevin A.
Date: Thu, 27 Apr 1995 20:24:01 -0500 (EST)
To: jmorton@mhs.mendocino.k12.ca.us
Subject: flight

Dear Naomi and Jessica,

To avoid too many quotations, let me first say that all of the
following information is taken from "The Birder's Handbook," by Paul
Ehrlich, David Dobkin and Darryl Wheye.

Birds fly by creating regions of different air pressure above and below
the wings, just like airplanes do. If you hold a strip of paper lengthwise
and blow along the top of it, you will see this principle in action. The
higher the speed of the air across the wing (or paper), the lower the
pressure on that surface. If you look carefully at an airplane wing, you
might notice that it is more curved on one side than the other. This
means that on the curved side, the air has to travel further than on the
straight side to reach the trailing edge of the wing. If it has to
travel further, it has to travel *faster*, so that it can keep up with
the air on the bottom of the wing. This difference in airspeed reduces
the pressure on the top part of the wing.

Birds produce the lift required to fly by moving forward. They do this
using specialized feathers called primaries and secondaries (you may
already have learned about these feathers). When the wing is flapped
downward, the primaries act like an airplane propellor; the feather is
angled so that the low pressure side of the feather is toward the front
of the bird. This pulls the bird forward. The secondaries also help in
causing lift, especially on the upstroke of the wingbeat. On the
upstroke, when not as much lift or forward motion is generated, the wings
are folded close to the body to reduce drag.

Obviously, a bird needs some serious muscle-power to make this work.
In fact, in hummingbirds, up to 30% of the weight of the birds is devoted
to flight muscles. It requires a tremendous amount of energy for a
hummingbird to hover, which is why you will frequently see them resting
in between trips to a feeder. In addition, birds are well adapted for
flight. The bones of many birds are hollow, to eliminate weight but
remain stable and strong. Diving birds, however, like penguins, find no
advantage in hollowed bones, since the weight helps them get down to
where their prey are. Another factor in how birds are able to fly is
their respiratory system. Birds' respiratory systems are much larger and
more efficient than ours are, since flying is much more demanding than
walking or even running. Many bird lungs are specialized so that when
their breath is "double-processed". They have two chambers in their
lungs, so they can get as much oxygen from the air as possible. In fact,
their breathing system is so efficient that the inhaled air rarely mixes
with the air to be exhaled, which would dilute the oxygen content of the
inhaled breath. The muscles Naomi mentioned are also connected to the
respiratory system, in that the flight muscles are red in color, due to
the presence of chemicals which carry oxygen to the muscle. (This is
known commonly as "dark meat"; non-flight muscles are lighter in color -
"white meat").

Wing shape also has a lot do with how birds fly. The ratio of the wing
length to wing width is called the "aspect ratio". Birds with a low
aspect ratio, such as doves, woodpeckers, and other birds of the
order Passeriformes (50% of the worlds birds are in this order), need a
quick getaway and rapid, twisting flight, but not so much sustained
high-speed flight. Their low aspect ratio wings give them these
abilities. Birds which must be fast and agile, like Falcons for example,
have wings that taper to a point, for maneuverability. Slots between the
feathers on the tips of the wings help smaller birds, like warblers, be
agile as well. An interesting exercise is to compare the wing shape of
similar birds which live in different habitats; for example, a forest
hawk and a field hawk. The forest hawks have a lower aspect ratio for
maneuverability and short-distance speed, while the field hawks have a
larger aspect ratio for attacking speed and hovering.

These are a few of the reasons birds fly and other animals don't.
Flying fish don't really fly, they glide. The same is true of flying
squirrels. The ability to fly in birds is the product of millions of
years of evolution, and in man the product of a few years of thinking by
the Wright brothers. Perhaps you have read the story of Icarus, the man
who tried to fly by gluing feathers onto his arms with wax. When you
read that story, "The Flight of Icarus," think about these and other
reasons why birds fly and we don't.

Finally, I'm sorry it has taken so long to get a response back to you. I
am finished for the year, except for my final exams, which start in about
10 days. I will be in Panama, taking a much needed vacation for three
weeks starting May 10, and I'll be back on email by June 1. There will
be much great birdwatching inbetween!

Sincerely,
Kevin A.