porting surface of their wings is very great, but for the smaller and 

 shorter-winged birds lack of buoyancy at high altitudes presents a diffi- 

 cult obstacle in flight. Even when flying close to the earth, small birds 

 have to keep their wings in rapid motion. 



Another postulate favoring the high-altitude flying theory was that 

 the wonderful vision of birds was their sole guidance during migratory 

 flights; and to keep landmarks in view the birds were obliged to fly 

 high, particularly when crossing wide areas of water. This will be 

 considered in greater detail under Orientation (p. 28), so here it will 

 be sufficient to say that birds rely only in part upon vision to guide 

 them on migration. Also, it is to be remembered that there are definite 

 physical limitations to the range of visibility even under perfect atmos- 

 pheric conditions. Chief of these is the curvature of the earth's sur- 

 face. Thus, if birds crossing the Gulf of Mexico to Louisiana and 

 Florida flew at a height of 5 miles, they would still be unable to see a 

 third of the way across. And yet this trip is made twice each year by 

 thousands of thrushes, warblers, and others. 



Actual knowledge of the altitude of migratory flight is scanty, though 

 estimates obtained by means of the telescope, and still more accurate 

 data resulting from altimeter observation from airplanes, are slowly 

 accumulating. It is, of course, obvious that some birds that cross 

 mountain ranges during migration must attain a great altitude. Ob- 

 servers at an altitude of 14,000 feet in the Himalayas have recorded 

 storks and cranes flying so high that they could be seen only through 

 field glasses. Being beyond the range of unaided vision they must 

 have been at least 6,000 feet above the observers, or at an actual altitude 

 of 20,000 feet above sea level. Such cases, however, are exceptional 

 as aviators have reported that they rarely meet birds above an altitude 

 of 5,000 feet. 



It is now known that migration in general is performed below a 

 height of 3,000 feet above the earth. Some proof of this statement is 

 available. Observations made from lighthouses and other points of 

 vantage indicate that migrants commonly travel at altitudes of a very 

 few feet to a few hundred feet above sea or land. Sandpipers, sander- 

 lings, and northern phalaropes, observed in migration on the Pacific 

 oceanic route, have been noted to fly so low that they were visible only 

 as they topped a wave. Observers stationed at lighthouses and light- 

 ships ofi the English coast have similarly recorded the passage of land 

 birds, which sometimes flew just above the surface of the water, and 



27 



