In general, wind speeds are stronger at mid-day than at night or early morning, and increase from the ground, where friction slows the wind, up to several thousand metres. In addition, the air mass in which birds migrate is continually changing in speed and direction, and birds must continually adjust their behaviour if they are to migrate to a predetermined destination in the most energy-efficient way. That birds respond to wind is shown by the frequent observations that: (1) they tend to depart only in favourable (following) winds; (2) they often select flight altitudes where winds are favourable; and (3) they compensate for wind drift, at least to some extent (providing that they can see the ground below).
The long-standing observational evidence that birds mostly set off with following winds has been confirmed for individual radio-tagged birds. On days when radio-tagged passerines departed from south Sweden, the tailwind component was significantly greater than on days when birds were present but did not leave (Akesson & Hedenstrom 2000). Similarly, radio-tagged Brent Geese Branta bernicla en route from the Netherlands to arctic Siberia selected for each stage of their journey days with wind assistance (Green et al. 2002), and the same held for Bewick's Swans Cygnus c. bewickii migrating from Denmark to northern Russia (Klaassen et al. 2004). The reluctance of birds to fly against headwinds is clearly adaptive, but can sometimes result in considerable delay. For example, in 1994 when winds were favourable, four radio-tagged Barnacle Geese Branta leucop-sis took 5-15 days to migrate between Svalbard and Scotland; but in 1995 when winds were unfavourable for much of the time, radio-tagged birds took 9-36 days, with five individuals arriving 10 days (three birds) and 24 days (two birds) later than the majority that year (Butler et al. 2003).
Wind assistance is like food or fat reserves: it is a resource that fuels migration. If a bird with a given flying speed was blessed with a tailwind of the same direction and speed, it could in theory fly twice as far on the same fuel and in the same time, or the same distance on half the fuel and in half the time. Yet a bird flying into a headwind of the same speed could make little or no progress, however great its fuel reserve. In practice, it seems from radar studies that birds flying overland do not behave in quite this way, but fly slower than usual with a tailwind, and faster than usual against a headwind (Chapter 3; Bellrose 1967, Alerstam & Gudmundsson 1999, Hedenstrom et al. 2005, Liechti 2006). It is as though they conserve energy when conditions allow, and expend more than usual when necessary. The net effect is that birds make slower progress than expected in a tailwind, and faster progress than expected in a headwind, but further study is needed to find how widespread this behaviour is. In any case, following winds reduce the energy cost of migration, which is important to many migrants. But where following winds exceed the birds' own flight speed, they also bring risks, especially over inhospitable terrain. Birds are then unable to maintain their preferred direction if the wind direction changes; and if their body reserves are exhausted or conditions deteriorate, they cannot easily return. Small-bodied, slow-flying species obviously face the greatest risks from adverse winds.
If the wind deviates to some extent from the birds' intended track, the bird can in theory correct for this by adjusting its heading so as to remain on track with respect to the ground (Figure 4.1). Birds do not then progress in the direction they are heading, but at some angle to it, which is closer to the intended track. The greater the crosswind component for a given flight speed, the greater this compensating angle must be (aircraft pilots refer to the angle between heading and track as the drift angle, a). This angle can be reduced if the bird flies faster.
2. Full drift A
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