8 THE RIDDLE OF MIGRATION 



serve it. Their temperature vacillates with that 

 of their surroundings. They are slaves to environ- 

 ment. Even the conservation of heat is not all- 

 sufficient, for when an animal exerts itself it gener- 

 ates heat at a greatly increased rate. Unless such 

 excess is eliminated as speedily as it is produced, 

 super-heating, fever and death result. Birds pos- 

 sess a non-conducting layer of feathers that ac- 

 counts for the conservation; they cannot curb the 

 output at times of extreme activity but they rid 

 themselves of the overflow through their lungs and 

 air sacs about as fast as it is produced. They en- 

 tirely lack the common mammalian safe-guard of 

 sweat glands. The essential point is that they suc- 

 cessfully maintain a relatively high temperature 

 (100° to 112°F.) under all conditions as long as the 

 food supply remains adequate. Most mammals, at 

 such temperatures, would be suffering a dangerous 

 fever. Unlike reptiles, birds thus become inde- 

 pendent of what would otherwise be an insurmount- 

 able bar to distribution. As things are it makes 

 little material difference to them if the surrounding 

 temperature is 150°F. in the tropical sun or -60°F. 

 (below zero) during the arctic winter. Not all birds 

 can tolerate this extreme range, the parrots and 

 their relatives, for instance, being able to resist but 

 comparatively slight degrees of cold. But to this 



