At the Project:
By now you've had weeks to witness the flying skills of Tree Swallows.  You've seen
them soar, dive, and dash about.  You've watched them bank, wheel, turn, pivot, and
hover.  You can't help but be impressed by their dazzling powers of flight.  Since they
truly are masters of the air, how can a web site about Tree Swallows not attempt a
page about flight?  Photo below by Homer Caliwag.

Make it a point when you visit your project to devote some time to simply watching
swallows fly.  Observe how they hold and move their wings as they execute different
maneuvers.  And watch how other bird species fly.  Compare their skills and techniques
with the swallows'.  How do you think body size, wing size, and wing shape influence
each bird's flying style?

To say flight is complex is a gross understatement.  Flight involves a great many
intricate interactions among a bird's nerves, muscles, tendons, skeleton, and feathers.  
Almost every aspect of bird anatomy and physiology has been extremely modified over
time as their flying ability increased.  But though people have admired bird flight for
thousands of years, it wasn't until the 1980's, when scientists were first able to make
slow motion x-ray movies of birds flying in wind tunnels, that they truly began to
understand just how all a bird's parts worked together in flight.  We caution you that the
content that follows barely scratches the surface of bird flight.  In our opinion by far the
best and most complete treatment we know regarding this fascinating subject is found
in the
Handbook of Bird Biology, 3rd Ed., from the Cornell Lab of Ornithology.

When you really consider it, flying seems an impossibly difficult thing to do.  To fly
successfully an animal must be able to resist the force of gravity and support itself in
air, a movable, flexible, and not very supportive medium.  And once airborne an animal
must be able to both power and control its movement.  Many animals can glide, but of
all the animals that have ever lived on earth only insects, pterosaurs, birds, and bats
have developed the ability to truly fly, evidence of flight's difficulty.  And to achieve
something this hard the bodies and metabolisms of these groups had to undergo
drastic modifications.

The most obvious adaptations for powered flight are wings.
  • Powered flight requires wings of some kind, flattened surfaces that can be
    pushed against air, providing upward lift, forward thrust, and continuous control
    of movement.
  • Beginning with the same basic ancestral forelimb bones pterosaurs, bats, and
    birds independently developed different types of wings.
  • Pterosaurs, the oldest group of flying vertebrates, now extinct, flew with "finger
    wings," with one finger of each hand extremely elongated and supporting a skin
    membrane reinforced by fibers.
  • Bats fly with "hand wings."  Their hands and fingers are greatly enlarged, and
    connected by a thin, tough web of skin that serves as a flight surface.
  • Birds developed a third kind of flying surface, "arm wings" (see below).  Unlike
    bats and pterosaurs, bird hand and wrist bones are reduced, not enlarged, and
    instead of irreplaceable skin membranes birds catch, move and ride air with the
    ultralight but strong, replaceable structures we call feathers.

  • Flying conveys advantages.  Flying animals can travel far to find the best living
    conditions at any given time.  They can fly from danger and can nest out of most
    harm's way.  And flying may help them find and catch food.  However, there are
  • For all three vertebrate groups the development of wings specialized for flight
    meant losing the use of these forelimbs for other tasks.

Feathers also show many adaptions for flight.
  • Feathers, the epitome of lightness, form a bird's main flight surfaces.
  • The large "flight feathers" of a bird's wings and tail help propel, steer, and
    balance the bird as it moves through the air.
  • The long flight feathers of the outer wing are called "primaries."  They attach to
    the bird's hand bones.
  • The wing's inner flight feathers are called "secondaries."  They attach to the
    larger of a bird's two forearm bones.
  • Together, the primaries and secondaries are called the "remiges."
  • Primary and secondary feathers are not fixed rigidly in one place.  Instead, they
    are free to twist when they are moved against the air.  
  • The "vanes" (the flat surfaces that protrude from each side of a flight feather's
    shaft) of the secondaries and primaries are not the same size.  The outer vane
    is smaller, the inner one broader (see below).  This difference becomes very
    important during flapping flight.

  • The smaller "contour feathers" on the bird's head, body, and wings smooth
    and streamline its shape, so it can pass through air with less frictional drag.
  • Some groups of these contour feathers, the "coverts," partially overlap the wing
    and tail flight feathers, adding to the bird's streamlining.
  • Try distinguishing these feather types in pictures on this web sites.

Birds have other vital adaptations for flight that aren't obvious to us because
they are internal.

Bird bodies are extremely modified for lightness and strength.
  • Over time many bird bones became smaller in size or were lost altogether.
  • Most remaining bones are thin-walled, hollow, and reinforced with tiny cross-
  • Modern birds have only three fingers, and two of these are very tiny.
  • Most bird wrist bones are fused together.
  • Many joints in bird spines, pelvis, and tails have been either eliminated or fused
    together for increased rigidity, lightness, and strength.
  • Bird shoulder bones are modified to execute the unique wing movements
    necessary for flight.
  • The main muscles powering flight are not out in the wings but concentrated in
    the chest and shoulders.  These flight muscles can make up an amazing 30-50
    percent of a bird's total body weight.
  • Bird sternums (breastbones) possess large flat "keels" where the flight muscles
    of the chest attach.
  • Modern birds have lost the teeth their ancestors once possessed.
  • Bird skin is very thin and light, but tough.
  • Bird reproductive organs are tiny, enlarging only during mating season, and
    modern-day female birds have only one ovary.

Different flight methods require different levels of energy.  For instance,
soaring, gliding and bounding, are less demanding than sustained,rapid
flapping.  Birds in general have adaptations that support more active
metabolisms than our own, such as:
  • Hearts that are five times as large as ours as a percentage of body weight, and
    can pump ten times or more faster than ours.
  • Many more oxygen-carrying red blood cells per ounce than non-flying animals.
  • Ability to maintain high body temperatures while active.
  • Digestive tracts adapted for rapid processing of large amounts of high-energy,
    fuel-providing foods.

But it's in their breathing and heat-regulating systems that birds are most
surprisingly and radically different from us.
  • Bird flight muscles may demand large amounts of oxygen to fuel powered flight.
  • This muscular exertion can generate large amounts of internal waste heat that
    must be removed from their bodies.
  • Birds meet these related needs using a combination of lungs and air sacs, a
    system possessed by no other group of living animals.
  • When we mammals inhale air enters our lungs and oxygen and carbon dioxide
    are exchanged in tiny dead-end sacs.  Then the used air is exhaled back out the
    exact same route.  Our lungs never empty completely so stale air being exhaled
    mixes with the next fresh breath, and we extract only a low percent of the oxygen
    available per breath.
  • Birds have much more complex and efficient respiratory systems, in which a
    bird's muscles and skeleton pump air through in a continuous one-way direction,
    not back and forth like ours.  Birds' one-way flow allows them to extract almost all
    the available oxygen from air inhaled.
  • Bird lungs do not have tiny dead-end sacs like ours for gas exchange.  Instead,
    air flows through tubes in bird lungs called "parabronchi," where oxygen and
    carbon dioxide are exchanged across the walls of microscopic capillaries.
  • And during a bird's breathing cycle air passes not just through its lungs but also
    through a complex system of interconnected "air sacs."  
  • Most birds have nine pairs of these thin-walled sacs.  The sacs fill parts of their
    body cavities, and even penetrate their muscles and bones.  A substantial part
    of a bird's body is actually filled with air.
  • As a bird flies its flight muscles and moving skeletal parts act like a bellows,
    rhythmically pumping air along through its lungs and air sacs.  
  • Although oxygen and carbon dioxide aren't exchanged in the air sacs, the
    routing of air through them is what permits a bird's one-way, rapid-ventilation,
    breathing system.
  • Air sacs have still another vital function: dissipation of heat.  Air flowing through
    the sacs picks up most of the heat a bird's muscles generate during flight and is
    conducted outside when the bird exhales.

Clearly bird bodies have been drastically modified for flight, but what about
flight itself?  How do birds do it?
  • Flight actually has three parts: takeoff from the ground or a perch, sustained
    powered flight, and landing, and each requires somewhat different actions by
    the bird.
  • The airfoil shape of bird wings helps.  Wings are more curved on top than on
    the bottom, which means air passing around has to go a bit farther and faster
    over the top.  This creates a pressure differential, with higher pressure below
    and lower pressure above the wing, that helps lift the bird's body up.

  • But, although airfoils alone are used to great advantage by gliding and soaring
    birds, they aren't enough for continuous powered flight, .
  • True powered flight also requires a direct input of muscular force to get a bird
    into the air, keep it there, and propel it in a controlled manner toward where it
    wants to go.

Taking off:
  • Taking off from a standstill requires an immense generation of force, and since
    the bird isn't moving forward through the air yet, it must use its own muscle
    power to lift itself up.  
  • A bird taking off typically makes a very exaggerated up-sweep, raising both
    wings high over its back (below left).
  • The wings are then driven downward with a long powerful stroke (below right)
    which lifts the bird into the air.

  • Birds on elevated perches can takeoff more easily, by simply dropping off the
    perch, spreading their wings to create lift (below), and letting gravity provide the
    initial forward momentum.

Sustaining powered flight:  
  • Once airborne, most birds must flap their wings at least occasionally in order to
    keep flying under their own power.  This may seem simple enough, but flapping
    is actually a very complex, multi-part action.  
  • Flapping involves downstrokes, upstrokes, and transitions in between.
  • And as birds flap they change the shape of their wings, pushing large wings
    down and raising small wings up.  This is an essential part of bird flight.
  • Note the various wing positions and relative surface area sizes among the
    migrating swallows in Don Binder's photo below.

Downstrokes (or Powerstrokes):
  • Most of the lift and forward propulsion of powered flight is generated on the
    downstroke, driven primarily by the "pectoralis muscles" of the chest.  The
    pectoralis are the largest and most powerful muscles in a flying bird's body.
  • At the start of downstrokes the wings are stretched out fully and extended
    forward, creating a large surface area (see below).

  • Next, the pectoralis muscles contact, pulling these "large wings" down against
    the resisting air.
  • As each wing thrusts down its flight feathers overlap.  The wider vane on each
    feather's shaft pushes more air and, because these flight feathers are free to
    move, each wide vane twists up until it meets the small vane on the next feather,
    shaping the wing into one big, broad, flat airfoil unit, as in the simplified cross-
    section below.

  • Each downstroke with the airfoil shaped wing propels the bird forward, with the
    strong flexible primary feathers providing most of the power and the secondaries
    providing most of the lift.

Upstrokes (or Recovery Strokes):
  • Once one downstroke is completed the next step is raising the wing back up and
    repositioning it so another downstroke can be made.
  • Wing upstrokes are powered mainly by the "supracoracoideus muscles,"
    which are located on the bird's chest underneath the pectoralis muscles, not on
    its back as one might think.
  • Birds have a couple of techniques that make their wing profiles smaller, to
    produce an efficient upstroke.  
  • Many birds flex or fold their wings slightly on upstrokes (see below), reducing the
    wing's surface, so there is less area pressing against air.

  • Now, the asymmetry of the flight feather vanes and the feathers' freedom to
    twist, which allowed them to form large wings on the downstroke, come in handy
    again.  On upstrokes wing flight feathers pivot, creating many open slots air can
    pass through with little resistance.

  • The swallow below shows both wing flexing and slots between its primaries as it
    upstrokes with "small wings."

What's the tail's main role in flight?
  • Bird tails don't flap.  In flight they coordinate with the wings or operate semi-
    independently, depending on circumstances.
  • During steady, straightforward flight the tail doesn't have much to do.
  • But when turning is required or air turbulence causes flight instability,
    movements of the stiff tail feathers, technically called "rectrices," help the bird
    steer like a rudder and maintain directional control.  

  • Eventually of course flying birds must land, and to land successfully they must
    slow down dramatically while maintaining enough control to avoid crashing.
  • Many birds accomplish this by making deep wing beats and fanning their tails to
    decelerate, then throwing their legs forward to absorb impact and contact the
    landing surface.  Photo below by Don Christian of SouthernShots21.

  • Some situations allow birds to swoop up as they land (see below), which lets
    gravity help slow their forward momentum.

Questions for the next Topic:  Mating in Tree Swallows.
DNA studies of Tree Swallows show the nest female is almost always the mother of all
the young in her nest, but the nest male often is
not the father of all young in his nest.  
What's going on here?
  • Are some females being forced to copulate with other males, and if so, why
    would males want to do this?
  • Are some females "cheating" on their mates, and if so, why would females want
    to do this?
  • What could a resident male do to make sure he will be the father of the young in
    his nest?

Bird Flight
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