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Figure 4.2 Optimal use of a crosswind by a migrant. The bird is drifted by the wind during the initial stage of migration and later overcompensates to reach its goal. By invoking different altitude use and varying prevailing winds, variations of this model can be used to test predictions on real birds in the field (see text). From Alerstam (1979).

Figure 4.2 Optimal use of a crosswind by a migrant. The bird is drifted by the wind during the initial stage of migration and later overcompensates to reach its goal. By invoking different altitude use and varying prevailing winds, variations of this model can be used to test predictions on real birds in the field (see text). From Alerstam (1979).

at low altitudes, as they approach the end of their flight. By flying high at the start they gain the advantage of faster winds at those altitudes, even though their direction is not ideal, and make the correction later in the flight under weaker winds that prevail at lower altitudes (Figure 4.2). Alternatively, if winds shifted direction predictably along a migration route, as is common at latitudes 25-35°N, birds could allow themselves to be drifted in one direction at the start of a flight and in the opposite direction towards the end, as seems to occur in some overseas flights (see later).

Transoceanic migrants have no benefit from obvious landmarks, making it difficult or impossible to correct for drift. At take-off, such birds are clearly sensitive to local wind conditions, but once over the ocean they usually show little or no compensation for wind drift. While birds over land might settle and re-assess their position every few hours, transoceanic migrants may sometimes fly for more than 100 hours without apparent reference to local conditions. Their orientation, as followed by radar, may be largely explained by the maintenance of a simple compass heading, giving much more uniformity in directions than is usual among migrants over land (Williams & Williams 1990). However, as they descend towards the end of their flight, they begin to respond again to local weather and topography. Selection of appropriate weather cues before departure, together with simple vector migration, normally appear sufficient to allow such transoceanic migrants to make landfall on distant coasts or island chains.

Radar observations have been used to explore not only the extent to which migrating birds can compensate for drift by crosswinds, but also how much the prevailing circumstances (wind strength, visibility, day or night, over land or over sea, and cloud cover) affect the response. Patterns of compensation, full lateral drift, partial drift and downwind drift have all been observed, depending on conditions. Birds can presumably 'know' they are being drifted off course only when they have reference to some stable feature, such as the ground below; drift would have to be substantial before it could be detected from celestial or magnetic cues. These considerations may explain why compensation seems usual in low-altitude (<1 km) diurnal migrants, but is much less frequent or complete at high altitudes, and why it is more frequent in light winds (up to 5 m per second) than in stronger ones (Bruderer & Jenni 1990). In any case, it is rarely complete in nocturnal migrants and over large water bodies (Richardson 1990). Whether birds compensate for wind may reflect not only their ability to do so, but also their remaining body reserves, which influence the relative advantages or disadvantages of doing so. In some circumstances, it may be more economical for landbirds over water to drift downwind than to use scarce fuel reserves to fight against an unfavourable wind in order to remain on track. These considerations help to explain the variations in bird behaviour found with respect to wind (for behaviour of a radio-tagged Peregrine Falco peregrinus in a hurricane see McGrady et al. 2006).

Despite the effects of crosswinds, some birds can maintain remarkably straight tracks on migration. During a radar study of migrating waterfowl in Denmark, individual flight trajectories of bird flocks showed zig-zag patterns rather than straight lines (Desholm 2003). Analysis of these continual small changes in flight directions, which were too small to be detected by satellite, showed that geese and Common Eiders Somateria mollissima were flying, on average, only 0.7% and 1.6% longer distances than if they had flown along exact straight lines. The frequent small changes in flight direction could have resulted from birds repeatedly compensating for wind drift as they travelled towards their goal.

But what are the overall advantages in birds paying so much attention to wind? In conditions prevailing in central Europe in autumn, a bird migrating only on nights with favourable wind can, on average, increase its flight speed by 30% compared with an individual that disregards wind (Liechti & Bruderer 1998). Selecting the most profitable flight altitude may result in an additional 40% gain in flight speed. In other words, by responding appropriately to wind conditions, a bird can greatly increase its flight speed and greatly reduce its energy use during a journey through this region. The time needed for refuelling decreases accordingly, or the safety margins provided by the body reserves are extended. The bird is also at less risk of being blown far off course. Clearly, there are great benefits to a migrant in responding appropriately to wind.

For birds undertaking long non-stop flights over the sea, selection of favourable wind conditions at departure could be even more important than for birds over land. For example, many birds fly from northeastern North America directly over the Atlantic to Caribbean Islands or South America (Williams & Williams 1990). In autumn, the passage of a cold front brings favourable south-southeast winds and triggers departure. Once over water, as radar has shown, the birds continue flying south-southeast. When they encounter the trade winds near 25°N, their tracks are drifted to the south and south-southwest as they pass over the Caribbean. This drift is essential if birds are to reach South America, unless they redirect their heading en route. By not compensating fully for wind drift on the first part of their journey, the birds realise a faster and more energy-efficient migration to latitudes where wind direction changes, which then allows them to compensate for drift in an energy-efficient manner. They are able to follow the pattern in Figure 4.2, making no obvious compensation because crosswinds change predictably from one direction to the opposite direction along the route. At the time of spring migration, the winds are in the same direction as in autumn, yet the birds must fly in the opposite direction. They therefore make their northward journey to the west overland so that their entire two-way migration follows a clockwise loop.

Likewise, many birds in western Europe make their southward journey through Iberia into Africa, but make the return northward journey further east through Italy, so that in this case the entire return journey follows an anticlockwise loop. Some birds may have evolved elliptical (loop) migrations partly to minimise the effects of adverse winds, including latitudinal changes in wind direction. This is one of several hypotheses on the evolution of loop migrations, applicable to seabirds as well as to landbirds (Chapter 22).

With respect to wind, soaring birds provide a partial exception to the usual patterns. Updraughts strongly reduce the energy cost of migration for such birds, which often fly in side-winds or light opposing winds if updrafts are present. Thermals develop in calm or light wind conditions, but not in strong winds, which suppresses raptor migration in some regions, regardless of wind direction. Soaring birds also show no particular tendency to migrate on cold days in autumn, as do many other birds, probably because the necessary thermals develop best on warm days (Alerstam 1978a, Kerlinger 1989). An example of the effects of autumn weather on the migrations of different types of raptors is given in Figure 4.3. At the locality concerned, raptor migration occurred almost entirely within visual range, and so could be recorded accurately. Nevertheless, different species favoured somewhat different conditions for migration, depending on their particular flight modes.

Wind conditions are clearly crucial to successful long-distance migration, and the reactions of migrants to weather systems can be viewed as adaptive, ensuring a more energy-efficient and safer journey. The huge day-to-day fluctuations in the volume of bird migration reflect the continual adjustment of bird behaviour to prevailing conditions (Alerstam 1981). Some long non-stop flights of birds may be accomplished only with the aid of a following wind. This was the conclusion of several studies in which the known energy reserves of migrants were compared with their estimated needs on migration (for passerines see Wood 1982, Izhaki & Maitav 1998a; for shorebirds see Stoddard et al. 1983, Piersma & Jukema 1990, Marks & Redmond 1994, Tulp et al. 1994, Butler et al. 1997). However, these findings may be modified as more information becomes available on the energy needs of migrating birds (Chapter 5: Appendix 5.1). In some species, wind assistance has been found necessary for birds to achieve their migrations in the time observed (Butler et al. 1997), and in other species, arrival dates have been related to the proportion of days with following winds during the normal migration period (for Bewick's Swans Cygnus columbianus see Rees 1982, for Lesser Snow Geese Chen caerulescens see Ball 1983, for Barnacle geese Branta leucophrys see Butler et al. 2003). In order to better understand the variable relationships between behaviour and wind conditions in migrants, more research is needed, aimed particularly at testing predictions from mathematical models, such as those of Alerstam (1979).

Most of our understanding of weather effects on migration is based on radar studies which can provide less biased information on mass migration than any

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Sharp-shinned Hawk

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