My Ph.D. degree is in nutritional energetics
so I could write volumes on the subject.
I once gave a lecture at the University of
Missouri. I started by indicating how complex
the subjects of nutrition, thermal dynamics
and other aspects of nutritional energetics
were. With only a 50-minute class, I said
that I didn't know where to begin. A student,
from the back of the room, said "start
near the end." I will now take that
advice.
Birds, like humans, are homeothermic (warm-blooded).
In homeotherms, the heat produced by muscular
exercise, metabolic activity, and absorption
from the environment must equal the amount
of heat lost by the individual to its environment.
In temperate zones, like Utah, the environment
is generally cooler than the bird's body
temperature, so food energy (calories) is
required to compensate for energy lost to
the surroundings. In addition to the calories
needed to maintain body temperature, birds
need food for flight, egg production, feather
growth and other activities.
Heat is lost to the environment by the processes
of radiation, conduction, convection, and
the evaporation of moisture. Although birds
do not possess sweat glands, they do lose
heat by evaporation from moisture on the
skin and by panting. The ambient temperature
and relative humidity plays a role in how
much heat is lost this way.
The transfer of heat by radiation, conduction,
and convection always goes from a higher
temperature to a lower one. To moderate and
control this heat transfer, birds have developed
complex thermal-regulating behavior as well
as excellent layers of thermal insulation
-- fat and feathers. Radiation requires no
contact and is the reason why the sun's rays
feel warm and why a room feels colder than
the thermometer reading when the walls and
floor are cold. Conduction transfers heat
by direct contact. Conduction is very rapid
if the temperature difference is large. This
is why a cold water bath makes us feel cold
all over very quickly and why it is not advisable
to lick a frozen object. Convection occurs
when heat is carried away by the movement
of the air. Convection is the primary reason
"wind chill" readings are lower
than the actual (ambient) temperature.
The body temperature of birds is higher (41-42
degrees C.) than for most mammals (36-39
degrees C.). Body temperatures must be controlled
precisely as hypothermia (cold) will cause
death as will a body temperature of more
than 47 degrees C. -- just a few degrees
higher than normal.
How many calories are required to sustain
a bird? A lot! It takes a lot of food to
sustain life in a cold environment. Birds,
with their high body temperature and high
metabolic rate eat more food, ounce for ounce,
than most other vertebrates, shrews being
the exception. Birds tend to be very active
much of the time. Flying requires seven times
more energy than resting. Many birds migrate
thousands of miles, twice a year. Long distance
migrants, like Eared Grebes, will double
their weight at stopover sites like the Great
Salt Lake before continuing their migration.
Fat is an excellent fuel and most birds are
very efficient at converting excess food
to fat.
Acclimatization is the term given to the
changes, in behavior and body structure,
that occur during continuous or repeated
exposure to hot or cold environments. Birds
inhabiting Utah have adapted to the local
conditions here. Cover, food, and water are
essential daily requirements for them and
it is important that we humans be farsighted
enough to maintain habitats and food sources
for our feathered neighbors.
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