Start out with an inclined plane (your hand). Hold it against the wind
and experience the lifting force. First of all you get vertical lift and
then drag. It has some of the properties of an airfoil. Are there better
inclined planes than others? You can introduce camber to the plane. Camber
is the curve. A streamlined airfoil is actually the most efficient. Lift
is produced because the air flowing over an airfoil has a greater distance
to go, thus producing a higher velocity, and a lower pressure on
the top – resulting in a net lift.
Angle of Attack:
Another important aspect of lift is the angle of attack (defined as the
angle the wing is to the oncoming air force). At a certain angle of attack
the bird will stall and fall. This happens with planes if they try to
fly up to fast- they stall. What is the critical angle of attack anyhow?
This is a tricky questions because it depends a lot on airspeed. It also
depends on the type of wing a bird has. For instance, certain birds that
fly at low speeds, like vultures, have different types of wings with slits
in them that allow them to break up turbulence and gain lift. (Birds also
have an alula, or a mid wing slot that allows them to fly at different
speeds - similar to the flaps on an airplane wing).
Adaptive Evolution in the Avian Wing:
(Evolution. 11 212–224) As early as 1957, Savile, an engineer,
was able to examine the wings of birds to determine how the aerodynamics
of the wings work. In particular, how does air flow over the wing? For
any plane, air flows over the edges, causing turbulence that prevents
lift. Birds have been able to reduce the turbulent airflow at the corners
of their wings because they have wing-tip slots. Some birds have U-shaped slots,
some have V-shaped, and some are square (square ones
produce the most lift).
While lift is always an issue for birds, we have yet to talk
about thrust, or the act of moving the bird forward. Just flapping a wing
up and down isn't going to move it forward, just up. For birds, their
solution comes from the unique rotation of their wings. As the wings
beat, the primary flight feathers actually twist to provide thrust. This
twist is partly a result of asymmetrical veining of the feathers.
Types of Wings
- High Aspect Ratio Wing:
a. Much longer than it is wide. Has lot of vertical lifting area. Not
b. Not fast flying birds but more soaring birds
c. Albatross, or Frigate bird.
- Elliptical Wing:
a. Tends to be even pressure over most of the surface.
b. Usually found in birds that live in forests. … facilitates
a subtle change in the angle of the wing.
- High speed Wing:
a. Falcons, Plovers
b. Wings that are narrow and come out to a sharp tip. These wings reduce
drag. More of the wing-beat is producing forward thrust.
- High lift Wing
a. Extreme slotting in the wing-tips
b. Vultures, Eagles, etc.
Bones of a bird:
• Keeled sternum is very important in flight.
• Membranous portion around the wishbone (pectoralis muscle): Fits
on the craco-clavicular membrane, depresses the wing.
• Muscles that raise the wing are found below, attached to
the keel and the sternum- supracoracoideus. Originates on the
keel of the sternum. Has a tendon that passes through a hole in the coracoid
and attaches to the humorous.
• Classic study showed that the muscle of the supracoracoideus is
raised solely by raising the wings. In the 1930’s, scientists
cut the tendons on this muscle on crows and threw them out the window.
Turns out they flew well but they couldn’t take off again.
• The dorsal elevators are used for fine movements in the raising of
the wings. The crows mentioned above must have used these muscles to some extent.
• Marvelous convergent evolution in these divers
• Great auks and penguins use their beaks for flight only.
• Also the plotopterid … giant pelican
When one looks at bird flight, you can not overlook
those birds that have lost their ability to fly. In many ways, an examination
of flightlessbirds can help us answer some important
evolutionary questions. Here is a list of some examples:
• Cormorants on the Galapagos.
• Parrot in New Zealand:
• Owls flightless
• From Hawaii in the sub-fossil record there were a lot of geese.
• Hawaii had the only known flightless ibis. (nowhere else are ibises
This is a result of:
… Lost the keeled sternum.
... Coracoid and scapula have an angle greater than 90 degrees
... In birds that fly this angle is less than 90 degrees.
• Found on many islands of the world and many of them are flightless.
• Maybe the rails that can’t fly are just big babies (Neoteny).
• Flightless rails have the characteristics of baby rails that will
• Lots of foot-propelled divers
• Most extreme were Hesperornis.
It is believed that they were clumsy on land. Feet very far back
on the body.
• The femur can’t rotate downwards.
• It forms a hinge-like articulation
• Leg muscles of divers are much more incorporated into the body.
• Foot-propelled divers have a cnemial crest.
Grebes have toes with expanded surfaces lobes. Great adaptation for them.
- Tibiotarsus and tarsometatarsus are about the same length.
- If the two bones aren’t the same length it makes it difficult
to sit down.
- Center of gravity should be over the feet.
- You can compare two different postures.
Graviportal vs Cursorial.
• Graviportal Bird (Moa) has the bones right
on top of each other.
.... Not fast runners – convergent with that of elephants and large
.... Aepyornithidae (Aepyornis medius)
.... Dinornithidae birds.
• Cursorial Birds (Ostriches): important
for running. Their legs are built for running.
.... Cursorial means moving on land.
• Running birds have reduced numbers of toes. Convergent with
• Typical Avian birds typically have 3 toes forward.
• Perching feet have two toes forward and one toe back.
• Typical feet with big toes.
• Large toes provide greater surface area.
• Integrates locomotion with feeding.
Other toe adaptations
• Feathered toes increases the surface area by feathers on their
• Lark and Jacana (Larks walk on sand and the Jacanas walk on lily
• Under what circumstances do Mynas walk, hop and run?
• If they try to sit they stretch the tendons in their foot. Thus
as they sit their claws tighten on the perch.
• The sheath of the tendons have ridges and the shafts have bumps.
Thus as they sit, the tendons grip and don’t let go.
Locomote by climbing.
• There is a difference in the feet of birds that "climb up" and those that "climb down". Birds that climb up have a longer claw on the front toe. Birds that climb down have a longer claw on the back toe.
By Rob Nelson
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