North America rose during the course of their autumn flight over the western Atlantic to reach heights of 4-6 km over Antigua in the Caribbean, and later dropped abruptly as they approached South America (Figure 4.4; Williams et al. 1977). But radar has also revealed that birds crossing smaller stretches of water, such as the North Sea or the Great Lakes, lose height during the night, often flying within 100 m above the water surface, but rise again at dawn (the so-called dawn ascent, Myres 1964, Diehl et al. 2003). Thus, overwater echoes on the radar screen disappear late in the night, as birds descend below the radar horizon, and reappear towards dawn, an appropriate distance along the route. One view is that birds descend during the night in order to find a place to settle. If they find themselves still over water, they ascend at dawn probably to avoid predation by gulls, which become active around daybreak, and also to gain a better view (Bourne 1980). They may change direction to head towards land if they are within about 30 km (and can presumably see land).
Other altitude estimates were made by 'moon-watching' in some desert, foothill and mountain areas of central Asia (Dolnik & Bolshakov 1985, Dolnik 1990). The average flight altitudes were estimated at around 1 km above sea level over low desert, 1.5 km above sea level over foothills, 2-3 km over the Caspian, and 3.2 km above sea level over the Tien Shan and Pamir Mountains. In the latter areas, some birds reached 9.7 km above sea level, but flight altitudes at night varied greatly between different types of birds, with median values above sea level of 5.0 km in passerines, 5.8 km in shorebirds and 6.9 km in ducks (equivalent to 1.3, 2.1 and 3.2 km above ground). In general, these figures are in broad agreement with those found by radar in central Europe, and confirm that most movement occurs within 1.5 km of the ground, even though in mountain areas this takes the birds to heights of several kilometres above sea level.
Several species migrate regularly over the Himalayas, and at least one species over the highest parts, namely the Bar-headed Goose Anser indicus, which has been recorded at more than 8 km above sea level (Chapter 6). Demoiselle Cranes Grus virgo can reach altitudes of 7.5 km in crossing the Hindu Kush on migrations between their nesting areas in central Asia and wintering areas in northwest India. Clearly, such birds must be pre-equipped physiologically to avoid altitude sickness (hypoxia). Not all birds have such flexibility in their flight altitudes and, whatever the advantages of high-altitude flight, many species seem confined to migrate at low elevations, presumably for physiological or energy-based reasons, as in the swans discussed in Chapter 3.
Soaring birds are limited in flight altitude by the height reached by thermals, which is greatest around noon over land, and in most conditions seldom exceeds 1.5 km. During their migrations, soaring birds are continually rising and falling, as they climb in successive thermals and lose height between them (Figure 7.3; Leshem & Yom-Tov 1996a, Spaar & Bruderer 1996). Unlike many other birds, therefore, they cannot maintain constant altitude over long distances. Much higher altitudes have occasionally been recorded in raptors, such as a Bald Eagle Haliaetus leucocephalus which reached 4.5 km above ground in North America, but it can be assumed that such birds were travelling largely by flapping flight (Harmata 2002). Soaring seabirds, such as albatrosses, seldom reach more than 30 m above the waves. Clearly, not all birds are high fliers.
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