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BIRDING 101
LESSON 7: NUTRITIONAL ENERGETICS


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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