As mentioned already, migratory birds regularly cross high mountain ranges, often at extremely high altitudes. The high valleys may be snow-covered and frozen in spring, when they offer little or nothing for passing birds, but in autumn many birds can stop and feed in whatever vegetation is available there. The main problems are the considerable costs of climbing to high altitudes (greatest in large birds), and the low temperatures, thin air and low oxygen levels often encountered (Chapter 4).
The highest mountain range in the world, the Himalayas, sits right across a major north-south bird migration route, as do most of the European ranges, such as the Pyrenees, Alps and Carpathians. Depending on their primary direction, birds may cross high mountains or fly around them. Radar studies suggest that many birds cross mountain ranges in non-stop flights, but visual observations confirm that others are funnelled through valleys and passes, where they may stop and feed. For example, at the high pass of Col de Bretolet in the Swiss Alps, about 75 species have been found regularly to rest and feed, and many more occasionally. In effect, this amounts to almost all the small species that take this route.
Several species migrate regularly each year over the Himalayas, flying at more than 5 km above sea level, and at least one species over the highest parts, namely the Bar-headed Goose Anser indicus, which nests on high-altitude lakes on the Tibetan plateau, and winters in the lowlands of India. This bird has been recorded at more than 8 km above sea level, where temperatures sink below -50°C,1 and oxygen pressures below one-third of those at sea level. Taking advantage of the jet stream, these birds can achieve speeds in excess of 150 km per hour, and complete the 700-1000 km journey within a day. The species is physiologically adapted to high-altitude flight, having a large heart which beats unusually rapidly, and haemoglobin with an exceptionally high affinity for oxygen. It shows no increase in haematocrit (packed cell volume) or in haemoglobin concentration when exposed to simulated high altitude, and therefore avoids any increase in blood viscosity which could impair circulation (Black & Tenney 1980). Such birds can thus achieve feats of high-altitude performance that are shown by very few, if any other animals, and can do so without time to acclimatise. For instance, Bar-headed Geese Anser indicus may begin their spring migration near sea level, and reach altitudes exceeding 7 km in a few hours.
Tests have shown that Bar-headed Geese can remain conscious and stand erect in hypobaric chambers under simulated high-altitude conditions of slightly over 12 km (Black & Tenney 1980). This altitude far exceeds the tolerance limits of most mammals. Without acclimatisation, a person is in trouble at 4 km above sea level, but with training can reach more than 5 km without becoming breathless.
Clearly, high-flying birds must be pre-equipped with mechanisms which prevent altitude sickness (hypoxia) caused by oxygen deficiency in thin air. All flying birds may have this facility to some extent (Novoa et al. 1991), but species living or migrating at high altitudes need special adaptations which maintain oxygen delivery to the brain, gas exchange efficiency in the lung, oxygen binding by haemoglobin, cardiovascular performance and cellular function (see review in Berthold 1996). Some birds have an interesting adaptation in their blood, as they have at least two forms of haemoglobin side by side. These forms differ in amino acid composition, and in their oxygen carrying and releasing properties. One form acts as 'normal haemoglobin' for low altitudes, and the other as 'high elevation haemoglobin', which reaches greatest concentration in the blood of species that fly at especially high altitudes, thus ensuring adequate oxygen throughout (Heibl & Braunitzer 1988). In general among birds, lung ventilation is especially
1 Temperatures fall to —56.5°C at 11 km above sea level, and remain fairly constant with further elevation, even though air pressure and density continue to decline. This altitude marks the tro-popause, the boundary between the troposphere below and the stratosphere above. All migration occurs within the troposphere, and almost all within the lower half of the troposphere.
effective because of the air-sacs, and resulting continual flow of air through the lungs. Needless to add, not all birds have such flexibility in their flight altitudes, and many species seem confined to low elevations for physiological or energy-based reasons (Chapter 4).
It is not only the altitude that presents problems in mountains, for strong adverse winds are often funnelled through the valleys. One well-known migration route in Nepal includes the Kali Gandaki Valley, which provides a useful corridor through the Himalayas, with passes reaching up to 6.5 km. This route is used each autumn by up to 50 000 Demoiselle Cranes Grus virgo, flocks of which start their daily migration as soon as thermals develop in the mornings, soaring to heights that enable them to cross the passes. However, time is against them, for by mid-morning immensely strong headwinds develop, against which the cranes can make only slow progress. They also experience attacks from Golden Eagles Aquila chrysaetos, spaced every few kilometres along the valley. Compared to other cranes, demoiselles have one of the most difficult migrations, as they pass through desert and high mountains which offer very few feeding or safe roosting sites. As revealed by radio-tracking, they accomplish their migration relatively quickly, crossing both desert and mountains within one week (Kanai et al. 2000).
Not surprisingly then, many birds fly around mountains rather than taking a more direct route over the tops (Bruderer 1999). Most short-distance migrants, flying northeast-southwest in autumn, tend to circumvent the Alps. However, longer distance migrants en route to Africa fly more directly north-south, at higher altitude, and mostly cross the Alps. However, both groups fly lower in headwinds, smaller species being more affected than larger ones, and hence being more often deflected by mountains from their most direct route.
Migrants captured in high alpine passes often have more fat and longer wings than conspecifics caught in nearby lowland, implying that the two groups derive from different populations, one breeding further north than the other. This view is supported by ring recoveries. For example, Robins Erithacus rubecula occurring in the Alps during autumn migration originate from further north (Scandinavia) than those occurring in the nearby lowlands, which originate mainly from Germany and Czechoslovakia. Similarly, northern populations of Garden Warblers Sylvia borin (identified by their greater wing lengths) are more likely to cross the Alps than mid-latitude birds (Bruderer & Jenni 1990). Such differences presumably arise because longer distance migrants from higher latitudes migrate at higher altitude, or have been longer on the wing, giving more time to attain high altitude by the time they reach the Alps. Among trapped samples of Garden Warblers, the proportion of long-winged individuals increases on days with headwinds which induce lower flight than on other days.
Wetland and soaring species tend to avoid the higher passes. Raptors which migrate mainly by flapping flight, such as falcons, appear in the high Alpine passes much more often than those that depend on soaring, such as kites and buzzards. The few Common Buzzards Buteo buteo flying in the high Alps occur early in the migration season, when soaring is most readily possible. The average climbing rate of soaring birds decreases by one-third over the autumn migration season because of decreasing lift in thermals, as temperatures cool towards autumn (Bruderer & Jenni 1990). Hence, any tendency of birds to cross the Alps varies with the innate directions, flight capabilities and physiological state of the migrants, and with local weather conditions at the time. These findings from the central European Alps may well apply also to other mountain areas at similar latitude. In the New World, by contrast, most of the mountain ranges, particularly the Rockies and the Andes, run approximately north-south, so do not need to be crossed or circumvented by most lower ground migrants travelling north-south.
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