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U. S. NATIONAL MUSEUM BULLETIN 217 



bird of 1000 grams would utilize 84 k.-cal. in 24 hours at the resting 

 rate. In case the two sizes of birds metabolize substances of equal 

 value for energy, the bird weighing 10 grams would consume a given 

 percentage of body weight about 6 times as fast as the bird weighing 

 1000 grams. 



In order to relate the conversion of energy with the substances which 

 are actually metabolized it is necessary to define the energy provided 

 by the substances which are oxidized in the course of metabolism. 

 Fat has the highest value for energy production and its storage and 

 use involve little water. In the metabolic oxidation of fat, one gram 

 yields 9.4 k.-cal. (Brody, 1945, p. 35). A bird weighing 10 grams 

 would accordingly obtain 9.4 k.-cal. by the oxidation of 10 percent of 

 its substance in the form of fat. The oxidation of a given weight of 

 protein or carbohydrate produces only about half as much energy 

 as is derived from fat. These lower yielding substances are further 

 less suited than fat for metabolic reserves in that they are likely to 

 be stored and used along with a certain amount of water. 



The comparison of metabolic rates just given was based upon the 

 condition of animals resting in air warm enough so that no special 

 metabolic provision would be needed for maintaining the warmth of 

 the body. These metabolic rates are accordingly at the minimum at- 

 tainable experimentally for brief periods and they do not make pro- 

 vision for the usual activities of life or for cold weather. Reasonable 

 estimates can be prepared to indicate the magnitude of the daily me- 

 tabolism of birds migrating during the cool season when they reach 

 the arctic. These are shown in table 17. Because of the limited 

 natural insulation of birds of small size, at just freezing temperature, 

 a bird weighing 10 grams would need at least double its basal metabolic 

 rate in order to preserve its warm body temperature (Scholander, 



Table 17. — Estimate of the daily budget of metabolism of three sizes of birds 

 approaching the arctic terminus of migration in 0°C. average temperature 





Relation to normal basal 





metabolic rate 



Item 











Size 10 



Size 100 



Size 1000 





g. 



g. 



g. 



Sleeping (10 hours) 



20/24 



10/24 



10/24 



Waking, but not very active (10 hours), 2 









times basal rate 



20/24 



20/24 



20/24 



Flight (4 hours), 6 times basal rate 



24/24 



24/24 



24/24 



Sum of metabolic periods 



64/24 



54/24 



64/24 



Relation to basic metabolic rate 



2.67 



2.25 



2.25 



Kg.-cal. per day 



12.8 



45.0 



180 



Equivalent grams of fat 



1.36 



4.8 



20 



Reserve (days) provided by metabolism of 









10% of body weight as fat 



0.74 



2.1 



5 



