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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 5 PHYSIOLOGY
I.
II.
Introduction.
 A central theme in the success of life on Earth is the capacity for living organisms to
maintain homeostasis -- that is, to exert work (physiological or behavioral) and to
have adapted (via natural selection) to maintain conditions favorable for survival.
The ability of birds to maintain physiological homeostasis is particularly evident in
the way their behaviors and physiology enable them to maintain favorable O2 and
CO2 concentrations in the blood (respiration and circulation), regulate body
temperature, and regulate bi-products of metabolism (excretion and electrolyte
regulation).
Endothermy and Metabolism.
A. Birds are endothermic, maintaining a high body temperature around 40 C (higher
than most mammals). This high temperature allows high levels of sustained activity,
even when ambient temperatures are low; birds live in the earth's hottest deserts to its
coldest seas.
B. To maintain this high temperature, birds "pay" an energetic cost. This cost is their
basal metabolic rate (BMR), which is the body activity rate required to power the
body under normal resting conditions. Metabolic rate is elevated for various
activities. For example, flying raises metabolic rate to about 10-25 times that of
BMR.
C. Scholanders' model of endothermy.
1. Scholanders' model of endothermy has become a classic way to study
metabolism and temperature regulation. A bird's thermoneutral zone (TNZ) is the
temperature range over which it has little trouble maintaining temperature and
metabolism. DRAW
2. At temperature below the TNZ, a bird must take measures to elevate metabolic
rate and generate more heat (shivering, hunting, extreme fat metabolism).
3. At temperatures above the TNZ, a bird must also take measures to elevate
metabolism so as to cool (it takes work to cool) by evaporation (dilate blood
vessels, pant, gullar fluttering, etc.)
4. The shapes of these models vary between species in ways you might expect:
a. Surface Area to Volume Ratios (SAVOLRATS) are very important in
biology. Increasing body size results in decreasing SAVOLRAT
b. Small birds have high SAVOLRATS, so they lose and gain heat rapidly from
all their skin, so their TNZ is narrower and their BMR higher than larger birds
with lower SAVOLRATS. (with all these abbreviations, this is beginning to
read like a government document)
D. A useful simplified formula for the transfer of temperature from an animal to its
environment is this: Flux = T  c  SA; where flux is the rate of temperature
exchange, T is the difference between body and ambient temps, c is the coefficient
of conductance (insulation lowers c and slows flow), and SA is surface area over
which temp exchange occurs.
E. Birds have many morphological, physiological, and behavioral ways to adjust their
body temperatures when ambient temps are unfavorable. Usually these methods
affect one of the three variables in the flux formula. OVERHEAD
Condition
Adaptation or Behavior
Energetic Outcome
High Temperatures
In these cases, birds
need to LOOSE body
heat to environment
(increase flux)
1. Adjust feathers (compact them)
1. inhibits feather insulation, which
increases “c”, facilitating heat loss
2. Shunt/bypass counter-current
insulation
2. maximizes diff. b/w hot arterial
blood and air temp (higher T),
facilitating heat loss to environment
3. Find some shade (maximizes T)
4. increases SA of important
evaporative heat loss areas
5. increases SA for blood temp
exchange
3. Micro-habitat selection
4. Panting, gular flattering
If temps are so high
that they exceed body
temp….they need to
absorb LESS heat from
environment (decrease
flux)
Low Temperatures
In these cases, birds
need to SLOW the loss
of body heat to
environment (decrease
flux)
5. Expose areas with high
concentrations of near skin blood
vessels
6. Controlled hyperthermia
1. Adjust feathers (puff them up)
1. increases insulation, which
decreases c, fighting heat loss
2. Counter-current insulation
2. reduces T between warm arterial
blood and cold environment, allows
heat that is lost to go to returning
cooler venous blood; decreases heat
lost to environment
3. Find some sun (lowers T)
4. increases SA in small areas
which can absorb heat (this increases
flux to yield a positive flux of heat
into the body)
3. Micro-habitat selection
4. Orient toward sun & expose skin
5. Controlled Hypothermia/Torpor
Outside of TMZ
6. elevates body temperature in
hormonally controlled
manner…reduces T, helps save
water that would otherwise be lost
trying to cool the body.
6. Shiver
5. decreases difference between
ambient and body temperatures
(lowers T) – possible only when
inactive
6. increases MR and generates heat
III.
Getting O2 to and CO2 from tissues.
A. Birds have higher sustained metabolic rates than mammals. To support this activity,
they need to deliver O2 to lots of tissues very efficiently. This is accomplished with
sophisticated respiratory and circulation systems.
B. Respiratory system
1. Primary function is to exchange CO2 in body for O2 in air. Secondary function is
to aid in temperature regulation.
2. Birds lungs are VERY different from other tetrapods' lungs (like ours). Our lungs
are blind two-way air passages. When you exhale, you can't get rid of ALL the
air in your lungs, about 20% remains, called residual volume. This makes our
lungs inherently inefficient. Birds' lungs are better.
3. Bird lungs are made up of many thin walled tubes, called tertiary bronchi, plus a
bunch of air sacs (usually 9) in "strategic" places. OVERHEAD
a. Most gas exchange occurs in bronchi.
b. It takes two "breaths" for a given volume of air to enter and leave a birds
body. OVERHEAD
i.
In the first inhalation, O2-rich air flows all the way into the birds,
through the lungs, into abdominal posterior air sacs. A little gas
exchange occurs here.
ii.
In the first exhalation, this air (still rich in O2) flows forward into the
lungs, where it is forced into finer and finer branching bronchioles,
that eventually intertwine with blood capillaries and this is where the
majority of gas exchange occurs. Nearly all the inhaled air in this way
exchanges its O2 for CO2.
iii.
In the second inhalation, this air, which is now rich in CO2, flows into
the anterior air sacs, where just a little more gas exchange occurs, but
not much else happens (the air is just getting "out of the way", because
remember this inhalation also brings in a new volume of air that goes
past the lungs and into posterior air sacs).
iv.
In the second exhalation, the air (now very O2 depleted) is expelled
through the trachea.
c. Other functions for air sacs:
i.
Air bags for diving birds.
ii.
Mating displays (sage grouse, frigate birds).
iii.
Puts pressure on syrinx, enabling a greater range of notes for singing
birds than could be reached otherwise.
C. Circulatory system.
1. Those efficient lungs are only as good as the heart is at getting all that O2 rich
blood to the tissues that need it.
2. Birds have a 4-chambered heart like mammals, but it is relatively larger (about
twice as big) and delivers larger stroke volumes. This leads to extremely high
blood pressure; and a frequent cause of death in birds is aorta rupture. When it
works though, it is incredible. Birds are like Indy cars. They run their engines
close to the edge. Great performance, great risk.
IV.
3. 4 chambers, remember, separates the pulmonary circulatory system (that which
goes to and from the lungs) from the systemic circulatory system (that which goes
to the rest of the body).
Excretory System.
A. Like the lungs, the avian kidneys are a unique evolutionary departure from both
reptilian and mammalian kidneys.
B. Birds need to conserve water; they also can't afford to store lots of it to dilute toxic
wastes in a heavy bladder. So they excrete uric acid, not urea.
1. Metabolic activity produces nitrogenous wastes that can become toxic if allowed
to accumulate in the body. Many terrestrial animals dilute these wastes and store
them (as urea) until they can be excreted.
2. Not birds. They excrete wastes as a semisolid solution of uric acid crystals
(white). In this way, they require about 20-40 times less water then mammals to
excrete a given amount of nitrogen. Even the famed kangaroo rats that excrete a
dry-ish paste can't compare to that.
3. However, the cost is paid in an inability to concentrate unwanted electrolytes
(e.g., salt) in the blood. Mammals can do it (via loop of Henle) bird's can't do it
very well at all.
4. This is a problem especially for seabirds, which are forced to drink seawater to
get their water. So these birds, and others, have evolved special nasal salt glands.
5. These salt glands extract salt and other electrolytes from the body and excrete
them down the nose; some birds sneeze them out (storm petrels). "Tube-nosed"
birds have especially well developed nasal glands.
6. This active extraction of salts from water takes energy….seabirds’ BMR can
increase by up to 7% when their salt glands are actively pulling salt out of
seawater.