some birds must cross mountain ranges during migration and attain 

 great altitudes. Numerous observations have come from the 

 Himalayas (Geroudet 1954; Swan 1970). Observers at 14,000 feet 

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

 through field glasses. In the same area large vultures were seen 

 soaring at 25,000 feet and an eagle carcass was found at 26,000 feet. 

 The expedition to Mt. Everest in 1952 found skeletons of a pintail and 

 a black-tailed godwit at 16,400 feet on Khumbu Glacier (Geroudet 

 1954). Bar-headed geese have been observed flying over the highest 

 peaks (29,000+ feet) even though a 10,000-foot pass was nearby. 

 Probably 30 or more species regularly cross these high passes (Swan 

 1970). 



Except to fly over high mountain ranges, birds rarely fly as high as 

 those traveling down the western Atlantic (Richardson 1972). Many 

 of these birds are making long-distance flights to eastern South 

 America and beyond. Therefore, flight at high altitudes in this region 

 is probably advantageous for them. Richardson postulated stronger 

 advantageous tail winds were found higher up and the cooler air 

 minimized evaporative water losses. This investigator found air 

 temperatures averaged 35 F at 10,000 feet over Nova Scotia in 

 September. The lower the ambient temperature, the more heat can 

 be lost by convection and the less water is required for cooling. Also, a 

 bird flying high can achieve the same range as one flying at sea level 

 but must cruise at a higher speed with a corresponding increase in 

 power output and oxygen consumption. But the increased cruising 

 speed results in shorter flight time and less interference from wind 

 (Pennycuick 1969). 



Another postulate favoring the high-altitude flying theory was 

 that the wonderful vision of birds was their sole guidance during 

 migratory flights. To keep landmarks in view, birds were obliged to 

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

 considered in greater detail in the section, "Orientation and 

 Navigation," so here it will be sufficient to say that birds rely only in 

 part upon landmarks to guide them on migration. Also, it must be 

 remembered that definite physical limitations to the range of 

 visibility exist even under perfect atmospheric conditions. Chief of 

 these is the curvature of the earth's surface. 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 

 (during daylight hours). And yet this trip is made twice each year, 

 much of the distance probably at night, by thousands of thrushes, 

 warblers, and others. 



The altitude of migration depends upon the species of bird, 

 weather, time of day or year, and geographical features. Nocturnal 

 migrants, studied by radar, appear to fly at different altitudes at 

 different times during the night. Birds generally take off shortly 

 after sundown and rapidly gain maximum altitude. This peak is 

 maintained until around midnight, then the travelers gradually 

 descend until daylight. For most small birds the favored altitude 



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