BIRDS AND THE WIND McMILLAN 357 



when a moving air mass meets air moving in a different direction. 

 Just as eddies and rip tides are visible evidence of opposing currents 

 in water, so are many of our storms eloquent testimony of a battle 

 of winds. A bird changing from one current to another is analagous 

 to a man stepping from an escalator moving in one direction to 

 another moving in the opposite. There is a distinct shock which is 

 gone as soon as the change has been made. The bird, even though 

 he may be flying slowly, is relatively fast for his size and makes the 

 change quickly. Long dirigibles, slow for their size, have been liter- 

 ally sheared apart when straddling strong opposing currents. It is 

 possible that birds receive painful wrenches under like conditions 

 and endeavor to avoid them. 



Horizontal flight requires the most frequent change on days when 

 the currents are vertical. An ornithologist has made the guess that 

 migrating birds fly high on clear warm days because they are using 

 rising air to give them lift. It is the guess of the writer that they 

 fly high for the same reason that he does, to escape those currents. 

 Nature abhors a vacuum. Air rising from the surface must be re- 

 placed and on the days in question the replacement comes from aloft. 

 A "thermal," whether it is a steady stream or a series of bubbles of 

 hot air, resembles a fountain. The air goes up, but it also comes 

 down. For every updraft there are compensating downdrafts and 

 flight through them is a continual jarring and bumping. 



The upper limit of these convection, or mixing, currents is usually 

 marked by a layer of smoke and dust that has been carried aloft. 

 Above it there will be a horizontal current of air into which the ther- 

 mals cannot rise and where smooth flight can be maintained. The 

 highest elevation at which the writer has seen birds was at 7,200 feet, 

 just above the dust level near Port Arthur, Tex., where he passed a 

 flock of 20 or more in light brown plumage. At an air speed of 160 

 miles an hour he was upon them and had scattered and passed them 

 before they could be identified. The significant feature of the occur- 

 rence is that the birds were going with a 22-mile-an-hour wind out 

 of the west-southwest. 



What were the birds ? Whence had they come and where were they 

 going? The writer would very much like to know, just as he would 

 also like to know how, in April of this year, a lone goose happened 

 to be on top of a solid overcast riding a 30-mile-an-hour east-southeast 

 wind over southern Alabama. Snap judgment would say that the 

 goose was lost and bewildered. But he may have known exactly 

 what he was doing when he chose to ride a wind, above the clouds, 

 that was totally different from that miderneath. He may have 

 known that wind never travels in a straight line, that a small arc of 



