ALTITUDE OF FLIGHT AND MIGRATION 



The factors regulating the heights of bird migration are not clear. 

 High-altitude flight may be used to locate familiar landmarks, fly 

 over fog or clouds, surmount physical barriers, gain advantage of a 

 following wind, or maintain a better physiological balance. 

 Meteorological conditions probably account for most of the high- 

 altitude records. Wind conditions at ground level are usually quite 

 different in direction and velocity than at points higher up. 



In general, human estimates of bird heights are quite unreliable 

 except under special conditions, and these estimates will vary with 

 the eyesight of the observer. Lucanus (1911) found a European 

 sparrow hawk could be distinguished at 800 feet but disappeared 

 from sight at 2,800 feet. A rook (a European member of the crow 

 family) could be recognized at 1,000 feet but disappeared from sight 

 at 3,300 feet. Meinertzhagen (1955) did an interesting experiment 

 with an inflated model of a vulture painted black; it had a wing 

 expanse of 7 feet 10 inches. When released from an airplane at 4,700 

 feet, it was barely visible and invisible without binoculars at 5,800 

 feet. At 7,000 feet it was not picked up even when x!2 binoculars were 

 used. 



At one time students of bird migration believed normal migratory 

 movements took place at heights above 15,000 feet. They reasoned, 

 somewhat uncertainly, that flying became easier as altitude was 

 gained. It has now been shown, through comprehensive radar 

 studies, that 95 percent of the migratory movements occur at less 

 than 10,000 feet, and the bulk of the movements occur under 3,000 

 feet. However, birds can and do fly well over 15,000 feet without 

 apparent ill effects. The physiology of long-distance flight at high 

 altitudes is of great interest but can only be touched on briefly in this 

 discussion. 



Bird flight at 20,000 feet, where less than half the oxygen is present 

 than at sea level, is impressive if only because the work is achieved by 

 living muscle tissue. A Himalayan mountain climber at 16,000 feet 

 was rather amazed when a flock of geese flew north 2 miles over his 

 head honking as they went (Swan 1970). At 20,000 feet a man has a 

 hard time talking and running or other rapid movements are out of 

 the question; but those geese were probably flying at 27,000 feet and 

 even calling while they traveled at this tremendous height. 



Accurate observations on the altitude of migratory flights is 

 scanty, although altimeter observations from airplanes and radar 

 are becoming more frequent in the literature. An example is the 

 report of a mallard struck by a commercial airliner at 21,000 feet 

 over the Nevada desert (Manville 1963). It is, of course, obvious that 



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