362 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



such similarities between the movement of the wind and the move- 

 ment of the birds may be indications that the migrants are riding the 

 wind, but they are not proofs. 



Positive proof will be difficult to obtain, for it will be necessary 

 to know the starting and arriving points, the exact time and the 

 weather conditions over the entire route at the level at which the 

 birds fly. Most observations must, of necessity, be made from the 

 ground at the very point where conditions are the most likely to be 

 different from those under which the birds travel. Weather not only 

 has length and breadth but height, and the most rapid change is 

 vertically. Temperature, humidity, wind direction, and wind velocity 

 will all be different at 500 feet when there is but little change in miles 

 along the surface. Seacoasts are a vantage point for observations 

 and seacoasts are notorious for windshifts close to the surface. All 

 of this imposes a handicap to proving beyond question that whenever 

 possible birds migrate by riding the wind, but it is a handicap that 

 should work both ways. It should be equally difficult to prove that 

 they do not. 



The writer has been told that it is the general belief of bird stu- 

 dents that the contrary is true — ^that, while migrants prefer a quar- 

 tering wind to the rear, wind and weather are really of little concern 

 to them. If this belief is founded on a correlation of bird move- 

 ments with surface conditions a shadow of doubt can fall upon the 

 correctness of it. If an observer sees birds from the south land into 

 a cold northeast wind during a warm-front snowstorm, it would 

 be natural to assume that the headwind and the storm meant little 

 to the birds. But somewhere over that observer's head there is 

 certain to be air that has come from the same general region as the 

 birds. Contrary to what our heritage has taught us, snow does not 

 come from the cold, dry regions of the earth but from, the warm and 

 moist. The cold air at the surface is only the precipitating agent that 

 is forcing the warm air aloft and chilling it by expansion to the 

 point where it can no longer hold its moisture. As the average slope 

 of a warm front is 1 to 300, the birds can be 90 miles from the actual 

 front and still have a tail wind as low as 1,500 feet. 



Just as the observer could be wrong in his deduction while watch- 

 ing descending birds on stormy days, so he could be wrong while 

 watching migrants on clear days when one air mass is in full posses- 

 sion of his territory. The gradient wind tends to follow the isobars, 

 or lines of equal barometric pressure, while surface wind cuts across 

 them at an angle. The truth of this statement can be easily verified 

 by reference to a weather map as meager in details as that which 

 appears in our daily newspaper. Immediately off the surface the 

 wind begins to turn until between 1,700 feet and 3,000 feet it has at- 



