BIOLOGICAL ASPECTS 327 



Hock, Walters, Johnson, and L. Irving, 1950 ; L. Irving, H. Krog, and 

 M. Monson, 1955) but an arctic bird weighing 100 grams would prob- 

 ably be sufficiently insulated to get along at freezing temperature with 

 just the heat produced by basal metabolism. In table 17 I have al- 

 lowed 10 hours in the day for rest with metabolic rates on account of 

 cold, at twice the basal level in birds weighing 10 grams and at the basal 

 level in birds weighing 100 grams or more. I have then allowed that 

 during the 10 hours of the day when migratory birds would be awake 

 but not very active, they would require about double the basal rate 

 of energy expenditure in that period. I have allowed that during 

 4 hours of active movements of flying the metabolic rate would be six 

 times as great as the rate at rest, an expenditure of energy that cor- 

 responds with the rate of a man or dog walking rapidly. I have used 

 the figure, however, because Pearson's (1950) measurements showed 

 that the hovering flight of a hummingbird elevated its metabolism 

 to about six times the resting rate. 



The activity of a wild bird can hardly be regularly 50 percent 

 greater or less than I have allowed, for wild birds commonly pursue 

 their lives at a moderate pace. Even their swift movements in air, 

 which so far exceed the speed of man on the ground, are probably so 

 well within their capacity that they can probably fly as easily as a 

 trained man can walk. I have not noticed an arctic bird that appeared 

 to be exhausted by exertion, and it is my impression that they seldom 

 work at their highest rate. 



These estimates of daily metabolic costs allow us to evaluate the 

 significance of the reserves for nutrition which exist in the body of a 

 bird. If these reserves are in the form of fat and amount to 10 percent 

 of the body's weight they would provide the metabolic energy which 

 is expended during from % of a day to 5 days according to the size 

 of the common birds. 



The accumulation of fat which we acquire on our bodies is not 

 readily removed by current metabolism. The fat of obesity for ex- 

 ample is sequestered from current use and can only gradually be with- 

 drawn. But fat in the amount of 10 percent of the body weight does 

 not constitute obesity. I have found ducks two or three days after 

 they had been wounded to be in lean condition whereas ducks killed 

 instantly from the same flock were fat. It has been shown (p. 322) 

 that about a 10 percent decline in average weight is often visible among 

 certain species of arctic birds two or three weeks after their arrival 

 on their nesting grounds, and that (p. 324) there are numerous in- 

 stances of rapid decline in the weight of birds during starvation 

 which indicate that their substance can be quickly utilized for metabo- 

 lism when food is unavailable. Among the migratory birds after their 

 arrival at Anaktuvuk the amount of visible fat decreased as their 



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