356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



land," are frequent. Several ornithologists have predicated their 

 explanations of the way in which birds maintain their directions on 

 the false assumption that a flying bird can feel the pressure of the 

 wind. At least one has used mathematics to determine how much 

 lift a bird can get from winds of varying velocities when the wind 

 cannot strike a bird in the air. One authority has made the extreme 

 statement that "strong winds blowing in the direction in which the 

 birds are traveling are bad, since they interfere seriously with bal- 

 ance, and disarrange feathers." 



A flying bird, which is essentially a part of the wind, cannot be 

 struck by it any more than a man can be struck by the automobile 

 in which he is riding. To obtain a true picture it might be better 

 if the word "wind" could be discarded and "air current," "air stream," 

 or "air river" substituted. This nomenclature may make it easier 

 to understand that anything suspended in air cannot feel its move- 

 ment. A log floating in the water of a river has no pressure exerted 

 upon it. There is no water piled up in front. There is no wake. 

 Viewed from the shore the log may seem to move but it is the water 

 that is moving. In relation to the water, the log is stationary. The 

 same liquid surrounds it at all times. 



A balloon in the air is like the log in water. Although it may 

 move over the face of the earth at 50 miles an hour the flag on its 

 halyards will hang in a dead calm. It is the air that is moving, not 

 the balloon. A bird cannot float like a balloon but he can accomplish 

 the same result at the point where his own air speed is sufficient to 

 give him enough lift to balance gravity. He moves through the air 

 while the air itself moves, just as a boat on a river moves through 

 the water while the water moves. Like a boat, which will have the 

 same pressure on the bow for any set speed in any direction, a bird 

 will feel pressure on the forward part of his body in ratio to his own 

 air speed whether he moves upwind, downwind, or crosswind. 



Even if he rides a hurricane that is spinning at well over a hun- 

 dred miles an hour, the bird will feel not an ounce more of pressure 

 or have a single feather ruffled. So long as he stays aloft he will be 

 in no danger from the wind but an attempted landing would be cer- 

 tain to result in disaster. The bird must ride out the storm or reach 

 its calm "eye" before he can alight. As a possible explanation of 

 the many instances of birds being found in the "eye" it might be of 

 interest to note that the air of a hurricane spins inward and a bird 

 riding the storm for any length of time must inevitably be carried 

 to the center. 



Only in collision can moving air exert pressure. That collision 

 can occur with the earth and objects attached to it, or it can occur 



