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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 3 & 4 - FLIGHT I & II

Birds fly. Most of them anyway, and they do so in wonderfully spectacular
ways, and with an amalgam of sophisticated adaptations. Indeed, flight is
the central avian adaptation. Adaptations for flight are integrated
throughout a bird's body; the obvious areas include musculature, skeleton
system, and wing and feather shape. In this lecture and the next, we'll
explore these adaptations and discuss how they enable birds of different
sizes and shapes to fly with different abilities. In the following lectures,
we'll talk about avian digestion, reproduction, circulation, and excretion
systems, during which the concept of adaptations for flight will again
frequently be important.
I.
Avian skeleton. Bird skeletons are uniquely structured for flight.
Flight is physically stressful on a bird's skeleton, yet flight must
overcome the force of gravity. Thus bird skeleton evolution has
progressed in such a way as to maximize strength and rigidity while
minimizing weight, seemingly conflicting properties. Powerful and
delicate - the allure of birds.
A. Fusion, reinforcement, and reduction. OVERHEAD
1. Most bones and bills are hollow to reduce weight, bones most in
need of support have internal struts (wing bones) for
reinforcement. For most flying birds, combined weight of
feather exceeds that of bones!
2. Uncinate processes are bones in rib cage that connect adjacent
ribs to one another for added rigidity.
3. Synsacrum.
4. A bird's wing is homologous to other tetrapods' forelimbs.
However, the first, most proximate bone (humerus) is fairly
short and wide, making the bend of the wing somewhat nonintuitive. What we instinctively call the shoulder is actually the
wrist.
a. Shoulder and elbow joints are elongated for greater
flexibility, and are extra strong.
b. Many wing bones are reduced and fused to be lighter and
stronger. Most of the wrist bones are fused into a single
structure called a carpometacarpus.
c. Hand reduced to only 3 digits.
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II.
5. Keel is an enlarged sternum that anchors the large flight
muscles.
B. Pectoral girdle. A major skeletal adaptation for flight is the pectoral
girdle. Three bones which comprise the pectoral girdle (one set on
each side of bird) are the scapula, coracoid, and furcula.
OVERHEAD
1. The scapulae are on top of the rib cage, long and pointed
toward the back of the bird.
2. Coracoid attaches at front end of scapula, and is a thick bone
that stretches down to the keel.
3. The furcula is a thinner, more elastic bone that also extends
from the connection of the scapula and coracoid downward and
just anterior to the keel. The left and right clavicles together
comprise what we call the wish bone (furcula).
4. These three bones therefore form a sort of tripod of support,
centered on the bird's center of gravity and supporting the large
flight muscles and tendons.
Flight muscles. The two major great flight muscles - the pectoralis
major and the supracoracoideus - both originate on the pectoral girdle
and insert on the humerus. Thus, they both lie in the "belly" of the bird,
keeping its center of gravity low and centered.
A. Pectoralis major.
 Largest muscle complex, accounting for 15% of total mass of
your average flying bird. When this muscle contracts, it pulls
the humerus (and rest of wing) downward. The power stroke.
B. Supracoracoideus.
 Powers the recovery (upward) stroke of wing. Much smaller
and weaker than pectoralis. Originates on sternum, from which
a very strong tendon extends upward through the hole created
where the three pectoral girdle bones come together (called the
triosseal canal), up and over the coracoid (like a line going over
a pulley) and inserts on the humerus. When the muscle
contracts, the line (tendon) is pulled and the humerus is drawn
upward. This "pulley" system enables a muscle which draws a
limb dorsally to be situated on ventral side of body. Clever, no?
C. Muscle fiber types. There are two general types of muscle fibers.
Most muscles are a combination of both types, but in different
proportions.
1. Red muscle fibers. Dark meat.
a. Used for aerobic respiration (free oxygen).
b. Used for oxidative (requiring oxygen) work; most
appropriate for sustained work. Dark because they contain
lots of myoglobin and mitochondra. Many birds' flight
muscles are mainly comprised of red muscle fibers,
especially those that fly long distances -- duck breasts are
dark meat.
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III.
2. White muscle fibers. White meat.
a. Used for anaerobic respiration (no oxygen).
b. Used for rapid and powerful, but not sustained, work. Few
mitochondria, so lactic acid builds up quickly. Most
appropriate for quick bursts of flight. Birds that have short
explosive flights (fowl) have mainly white meat in breast.
Aerodynamics.
A. Basic forces. Four basic forces act on a bird that is trying to fly.
1. Gravity. The force of the earth's pull downward.
2. Lift. The force exerted on the underside of a bird's wing,
pushing it up. If lift  gravity, birds stays aloft.
3. Drag. The forces (there are several) keeping a bird from
travelling forward.
4. Thrust. The force a bird exerts by flapping its wings, pushing it
forward. If thrust > drag, the bird [accelerates].
B. Lift. How do birds generate lift?
1. Much of our understanding of bird flight comes from our
understanding of fixed wing airplane flight (which in turn was
in part modeled after birds).
2. A bird's wing is cambered, or curved, especially on the upper
surface. OVERHEAD As air hits and separates around the
leading edge of the wing, it gets compressed on the upper side
because the wing is curved. Air is a fluid just like water
remember, and a compressed fluid always flows faster than
uncompressed fluid (recall putting your thumb over part of a
hose's flow to make it go faster). Thus, air flowing over the top
of the wing flows faster than air on the underside. Bournoulli's
principle (details beyond scope of this course) states that fastmoving air imparts less pressure on adjacent surface than slower
moving air. Therefore, the pressure pushing down on the top of
the wing is less than that pushing up from the bottom of the
wing, so the net force is upward -- LIFT.
3. You can see from this that the more a wing is curved, the more
lift it will produce. A bird can behaviorally functionally change
the curve of its wing by angling the wing backward, changing
what in aerodynamics is called the angle of attack.
OVERHEAD
a. At large angles of attack, more lift is produced, so birds tilt
their wings backward to rise in the air.
b. At very large angles of attack, the differential between air
flow above and below the wing that the airflow separates,
turning the flow from what is called laminar flow (as in,
lamination of air over around the wing) into turbulent flow.
This fails to generate lift, and the bird stalls. Birds do this
on purpose all the time, such as when landing on a perch.
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C. Thrust and drag. To move forward, a bird's thrust must over come
drag (also called friction).
1. Two main types of drag.
a. Profile drag. This is the frictional drag caused by the shape
of the wing. It is reduced by streamlining. Same as for cars:
a truck is less "aerodynamic" than a corvette; it has higher
profile drag. The force of profile drag increases with the
speed of a bird.
b. Induced drag. Exists primarily at wing tips and is caused by
pressure differences between upper and lower wing
surfaces. DRAW Air flows around the tip of the wing from
high to low pressure, creating little eddies (like whirlpools).
Eddies act to slow a bird down. Induced drag is inversely
proportional to bird speed -- the faster it goes, the less
induced drag. Incidentally, one explanation for the reason
large migrating birds fly in V's is that they are riding each
others' wing tip eddies. DRAW
2. Thrust.
a. Birds generate thrust by flapping. The secondaries of a
wing act more like a fixed wing & produce lift (they're
proximal so they don't move too much in a "flap"); the
primaries are distal -- the provide most of the thrust.
b. Asymmetrical vanes in feathers roughly correlated with
flight strength. OVERHEAD
i.
The primaries are overlapped and stacked on top of
each other such that the a leading primary (higher
numbers) is beneath the one behind it. Thus, on the
downward stroke (power stroke), air catches the
large trailing vane of each feather and "locks" up
against the one above and behind it, forming a "solid
plate" that pushes air downward.
ii.
On the upward stroke (recovery stroke), air again
catches the large trailing vane and pushes in
downward, twisting each primary so that it slices
upward with little resistance (much like "feathering"
in crew).
D. Power curves. Since profile drag increases with air speed but
induced drag decreases with air speed, the power needed to fly
follows a parabolic curve. DRAW
1. Minimum power speed. The speed at which a bird is using the
least amount of energy is therefore at moderate speeds. Going
really fast or really slow costs more energy. Watch birds in
flight, you can see them "labor" at low and high speeds (whitetailed kites are good for this).
2. Maximum range speed is a little faster than minimum power
speed. Ever been low on gas? To get to the next gas station,
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IV.
V.
should you go really fast or really slow? Fast would get you
there quicker, saving fuel, but you burn a lot of it to go so fast.
Slow would burn less fuel per minute, but it would take so
long..... Well, the answer is somewhere in between. The details
are beyond the scope of the lecture, but might make an
interesting lab project. As it turns out, the maximum range
speed is a bit faster than minimum power speed. Studies of
Common Swifts in Europe have substantiated this theory.
When feeding, they fly 23 kmh; when migrating they fly 40
kmh.
Wing sizes and shapes.
A. Wing loading. The ratio of body mass to wing area is called wing
loading. It is basically a measure of how much weight the wings
are trying to lift and propel.
1. A high wing loading (small wings relative to body size) means
a faster flyer, because profile drag is minimized (small wings
are more "aerodynamic").
2. But remember small wings (high wing loading) don't
necessarily mean short wings. Albatrosses, which have the
longest wings in the world, have very high wing loading
because their bodies are so heavy and their wings are so narrow;
very little surface area on their wings.
3. Low wing loading generally means the bird is more
maneuverable (large wide wings relative to body).
B. Aspect ratio. This is the ratio of wing length to wing width. High
AR means "pointy."
1. A high aspect ratio reduces induced drag (the kind that forms
off wing tips) because wing tips are very narrow in high AR
wings.
2. High aspect ratio wings are best for fast flying -- not as
maneuverable.
C. Slotting. Slots are holes or cracks in the surface of a wing created
by asymmetrically shaped flight feathers and/or spaces between
feather tips. Slotting serves to increase lift, especially at low
speeds. OVERHEAD
1. Large soaring birds have slotted primaries that maintain laminar
flow at low speeds by allowing air to slip from the lower to
upper surface of wing.
2. Some other birds (some fowl) have slotting to maintain laminar
flows at high angles of attack, generating more lift at take-off.
D. Alula. The alula is group of feathers on leading digit (thumb) that
is controlled separately from the rest of wing. It also serves to
maintain laminar flow and create lift. OVERHEAD
Gliding flight (flight without flapping). Best done where conditions
create upward moving bodies of air, which help generate and maintain
lift and thrust.
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VI.
A. Thermal soaring. Vultures and many hawks "ride" rising columns
of air created by the sun heating the earth. OVERHEAD Vultures
also hold their wings in a positive dihedral position. This yields a
tendency for the body to right itself automatically and return to its
resting position. Watch vultures "teeter" as they soar. Wings best
for thermal soaring are large and wide, with substantial slotting.
B. Slope gliding, or dynamic gliding. Slope gliding is accomplished
by riding upward moving air that is deflected off of a surface.
1. Gulls can "sit motionless" in the air by riding upward moving
air off of a sea cliff.
2. Migrating hawks soar along mountain ridges, creating
traditionally used routes useful for counting hawks in migration
(Hawk Mt. in Penn. and the Marin Headlands).
3. Long-winged seabirds are especially skilled at slope gliding.
They can expertly glide along skimming the windward side of
an ocean wave for miles without flapping. If the wave crest is
not in line with where they want to go, they fly in a zig-zag
fashion (much like tacking into the wind in sailing) flying fast in
higher, faster moving air and accelerating into lower, slower
moving air where they use their momentum and the upward
lifting air off the waves to lift back into the faster moving air
again.
Flapping flight. A very different type of flight, one not much like a
fixed wing aircraft, is flapping flight.
A. Hummingbird flight. Hummingbirds are the epitome of flapping
flight. This type of flight is much more like a helicopter than an
airplane. OVERHEAD
1. When a helicopter's blades are rotating horizontally, each blade
acts like a wing and it creates lift upward.
2. When that rotational plane is tilted forward, part of the upward
force become forward force, and the blades produce lift and a
little bit of thrust simultaneously.
3. A hummingbird's wings do the same thing. OVERHEAD
4. When a hummingbird is hovering, it orients its body vertically,
unlike all other birds, so that its back is facing backward (not
up) and its belly faces forward (not down). So by flapping, its
wings go side to side (not up and down).
5. It generates lift because each feather acts like a blade of a
helicopter. On the forward stroke, each feather pushes a tiny bit
of air downward and creates lift. On the backward stroke, the
wing is twisted so that the leading edge remains the leading
edge, and now the "underside" of the secondaries are facing
upward. Again, each feather generates lift. This rapid back and
forth motion actually describes little figure 8's laying on their
sides (infinities I guess), much like we do when we tread water.
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VII.
6. As you can tell, both the forward and backward strokes in this
type of flight are power strokes; there is no recovery stroke. So,
as you'd predict, in a hummingbird the supracoracoideus (the
muscle that pulls the wing up, or back in the case of the h'bird)
is roughly the same size as the pectoralis (the downward pulling
muscle), it is not much smaller like it is in most other birds.
7. To fly forward, the hummingbird simply tilts this figure 8
forward by rotating the wings and tilting the body more
horizontally. Zoom.
8. To fly backward, the hummingbird tilts the figure 8 backward.
9. This work on hummingbirds was very helpful in that it
highlighted that there was more than one way to generate lift,
and now people are beginning to recognize the fixed wing
model for bird flight is too simplistic, and that all birds probably
have some elements of the hummingbird's "flapping flight" in
their design.
B. Rayner's work with vortex gaits. OVERHEAD
1. As birds fly, their wings actually move forward during the
downward power stroke.
2. This seems counter intuitive because they want to push air
backward. This is because on the downward stroke, each
primary feather acts like an individual wing, generating "lift"
perpendicular to its plane. So by being tilted forward much of
this individual feather "lift" is transformed to forward thrust,
just as the hummingbirds lift is made into thrust by tilting the
figure 8 forward.
3. The result are small vortex rings that are created behind the
wings with each power stroke. These vortex rings, shaped like
smoke rings, create lift and thrust simultaneously and are
probably important in the flight of birds with low aspect ratios
and slow flight (like pigeons).
4. Rayner also identified another type of vortex-flight, which he
calls gaits (like a horse has different gaits, except for birds,
different species have different gaits; the same species doesn't
change). This he called continuous vortex gait, in which
continual vortices (not intermittent vortex rings) are created
behind fast flying birds with low aspect ratios (like falcons).
Flightlessness
A. Cost-benefit. An important concept in biology is that of costs and
benefits. Flying is a great benefit; it allows birds to occupy niches
that are unavailable to other animals. But growing and maintaining
wings capable of producing flight is very energetically costly, and
where the advantages of flight have not outweighed the
disadvantages; flight has often been lost.
B. Generally, loss of flight is associated with geographic isolation and
an absence of terrestrial predators. Does this suggest that flight is
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VIII.
more important in evading predation than in acquiring food?
Flightlessness has evolved independently in at least 13 orders of
birds.
Other means of locomotion.
A. Non-air flight. Some birds use their wings for underwater flight.
1. Evolution follows a progression. All the following birds are in
Charadriiformes.
2. Gulls use their wings for air-flying only -- and very well too.
3. Common Murres use their wings for air- and underwater-flying.
4. Great Auk (and penguins in southern hemisphere) use their
wings for underwater flight only.
5. For these birds, lift is not much of a problem, since it is easier to
be buoyant in water than air (especially the very dense salt
water). So these birds use both downward and upward strokes
of their wings for thrust -- making them behave more like
flippers than wings. (supracoracoideus and pectoralis are similar
in size here too)
B. Swimming and diving.
1. Water is much denser than air, and requires a different shape for
streamlining. Swimmers tend to be long and cylindrical, with
the center of gravity near the rear. OVERHEAD
2. Legs are situated near the rear; better for paddling, but worse for
walking and running -- this idea of evolutionary trade-offs is a
pervasive one in biology.
a. Dabbling ducks have relatively large wings and forward
placed legs. This allows them to lift from the water directly
into flight by pushing hard upward with their feet and
flapping vigorously.
b. Diving ducks have high wing loading (smaller wings) that
are less of a liability underwater, and their feet are very far
back on the body, which helps them paddle water directly
behind them. So they are great divers. But they can only
take off of the water after a long "run" during which they
build up speed. The wings can't generate the lift to spring
off the water, and a hard push of the feet only send them
forward, not up.
3. Diving.
a. Divers have heavier, laterally flattened bones to be more
streamlined and to help them sink.
b. Many also have less water repellent feathers, enabling them
to become wet and heavy (cormorants are the extreme in
this area).
c. Can squeeze air from plumage and even air sacs to "sink"
without diving. Grebes do this as you approach.
d. Respiration physiology of diving birds is poorly known
compared to mammals, but high concentrations of
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oxyhemoglobin in blood and muscles of diving birds enable
them to survive without inhaled oxygen for longer than nondiving birds.
e. Most dives are short in duration, less than 3 minutes, but
loons can stay down for up to 15 minutes.
f. Most dives are relatively shallow too, less than 5 meters, but
records are up to 250 m for penguins, 180 m for Common
Murres. Data come from telemetry in former, fixed depth
gillnets for latter.
C. Running and walking.
1. Running birds have elongated legs and toes, but often reduced
toe number (ostriches only have 2). Record speeds are 70 kmh
for Ostrich, 32 kmh for Greater Roadrunner, etc. Adaptations
also include a narrowing of the pelvis, which enables birds to
run without waddling from side to side like a wide-pelvis duck
does.
2. Birds with wide pelvises (most non-runners) are better off
hopping than walking. They can't swing one forward by itself at
a time very well.
3. Runners and walkers often "bob" their head to help their vision.
Actually, the head stays perfectly still as the body moves
forward during each step, then rapidly snaps forward before the
next step. This is because an object, especially a moving object,
is easier to see from a fixed than a slowly moving position.
4. Sometimes the best flyers have evolved to reduce expensive leg
and foot development; hummingbirds and swifts have all but
useless feet. They don't call them Apodiformes for nothing.
D. Climbing.
1. Woodpeckers and similar species have suits of adaptations for
climbing on tree trunks including an unusual toe arrangement (2
forward, 2 back), stiff retrices (tails feathers) to be used as
props, and reduced keels to "hug" the trunk. An interesting lab
project maybe to compare the morphology of unrelated species
that feed similarly on tree trunks (e.g., woodpeckers vs. creepers
vs. black-and-white warblers).
2. Other birds used other devices to climb. Parrots use their bills,
and young hoatzin use small claws located on the bends of their
wings.