As they ascend from ground level, migrating birds face progressively changing conditions. Wind speed increases, while air density, oxygen availability and temperature decline. Once birds break through the cloud layer, however, they can escape from mist, rain and snow. Wind has a major influence on the height above ground at which birds fly, for it can change in direction as well as strength, affecting the choice of cruising height. This is most strikingly demonstrated by the trade winds, which give way higher up to the anti-trades blowing in the opposite direction. This situation allows birds to find following winds in both autumn and spring, providing they fly at an appropriate height. In southern Israel, for example, the wind shear associated with the shift from trade to anti-trade winds fluctuates roughly around 1.5 km above sea level. Correspondingly, birds studied by radar flew mainly below this level in autumn, and mainly above it in spring, when some reached heights of 5-9 km above sea level. In the low-level jet streams of this region, birds also achieved ground speeds of up to 180 km per hour (Liechti & Shaller 1999).
Waders leaving the West African coast for Europe in spring must climb rapidly to altitudes greater than 3 km. It is only at such high altitudes that tailwinds keep the flight costs within reasonable bounds, thereby enabling the birds to complete their 4300-km non-stop flight to the Dutch Wadden Sea coast (Piersma 1990). Similarly, radio-tagged thrushes tracked on migration in North America ascended as much as about 3 km each evening until they found suitable winds (Cochran & Kjos 1985). With weak winds, they flew lower and accepted some lateral drift. If winds at altitudes above 75 m had unfavourable head or side components, the birds returned to the ground and did not migrate. Their migration therefore involved adoption of a constant heading, and mitigation of wind drift mainly by altitudinal adjustment rather than by lateral compensation. In most regions, where little altitudinal choice in wind direction is available, birds can only wait for favourable conditions or change their route between autumn and spring.
Apart from wind, the main weather factor influencing the altitude of migration is the cloud base. Most birds fly below the clouds where they can see the ground. The height of the cloud base therefore limits the vertical spread of migration, and if the cloud descends, it compresses the stream downwards, so the average flight altitude decreases. In some situations, however, birds fly above the clouds, presumably relying entirely on celestial or magnetic cues for navigation, with no visual reference to the ground below. Migration streams can also be compressed laterally as they are funnelled through mountain passes, or concentrate along coasts, river valleys or other 'leading lines', when birds may fly at low levels even without cloud cover.
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