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 appears to be between 500 and 1,000 feet (Bellrose 1971), but radar studies have found some nocturnal migrants (probably shorebirds) over the ocean were at 15,000 or even 20,000 feet (Lack 1960b; Nisbet 1963b; Richardson 1972). Observations made from lighthouses and other vantage points indicate that certain migrants commonly travel at altitudes of a very few feet to a few hundred feet above sea or land. Sandpipers, northern phalaropes, and various sea ducks have been seen flying so low they were visible only as they topped a wave. Observers stationed at lighthouses and lightships off the English coast have similarly recorded the passage of landbirds flying just above the surface of the water and rarely above 200 feet. During the World Wars, broad areas in the air were under constant surveillance, and many airplane pilots and observers took more than a casual interest in birds. Of the several hundred records resulting from their observations, only 36 were of birds flying above 5,000 feet and only 7 above 8,500 feet. Cranes were once recorded at an altitude of 15,000 feet, while the lapwing was the bird most frequently seen at high levels, 8,500 feet being its greatest recorded altitude. Records of the U.S. Civil Aeronautics Administration show that over two-thirds of all the bird-aircraft collisions occur below 2,000 feet and practically none occur above 6,000 feet (Williams 1950).