Topographic influences

There are circumstances where a long roundabout migration route is of advantage, an end result in its own right, rather than a historical legacy or stage on the way to a more direct route. By taking a detour, birds may avoid crossing an area of unsuitable habitat, even though such a non-stop flight may seem within their capabilities. The detour may offer several benefits, such as: (1) reduced risk from adverse weather or predators; (2) reduced energy costs, despite the longer journey (if, for example, winds are more favourable); or (3) suitable habitat in which the birds can stop and feed each day, thereby saving on fuel transportation costs. From examination of a number of common detour routes taken by birds, Alerstam (2001) concluded that the latter benefit - the saving on fuel transportation

Figure 22.3 Development of long oversea migrations by progressive corner cutting. By this process, the sea-crossing is gradually lengthened, but the total journey is gradually shortened. Different stages in this process (1-5) are shown today in different species, and may be presumed to represent the steps through which the longest overwater journeys developed. Note that the long oversea flights off eastern North America are made mainly in autumn, most birds taking a longer overland route in spring when winds over the sea are less favourable (Chapter 4).

Figure 22.3 Development of long oversea migrations by progressive corner cutting. By this process, the sea-crossing is gradually lengthened, but the total journey is gradually shortened. Different stages in this process (1-5) are shown today in different species, and may be presumed to represent the steps through which the longest overwater journeys developed. Note that the long oversea flights off eastern North America are made mainly in autumn, most birds taking a longer overland route in spring when winds over the sea are less favourable (Chapter 4).

costs - was probably paramount in the evolution of some detour routes (and also loop migrations discussed below).

In Eurasia, the major obstacles to migration, such as the various mountain ranges, the Asian deserts and the Mediterranean Sea, run west-east, at right angles to the mainly north-south migration routes. While some birds migrate over these obstacles, others migrate to one side or the other to avoid them. This gives another reason, in addition to the glacial legacy hypothesis, for a separation of autumn migration directions in Europe and elsewhere between southwest and southeast. An examination of the departure directions of central European passerines revealed that 45 species start their migration in a southwest direction, 10 in a southeast direction, while three species show funnel-shaped migration toward Italy and the central Mediterranean. In addition, 13 species show a spread of directions between southwest and southeast, and 16 others show a migratory divide that to a greater or lesser extent separates migrants moving southwest from those moving southeast (Figure 22.4, Bairlein 1985b).

A migratory divide is largely imposed on many North American birds by the Rocky Mountains which run roughly north-south down the western side of the continent. In many species, birds breeding to the west of this range migrate south down the western side of the Americas, while those from the eastern two-thirds of the continent migrate mostly southeast, into the southeastern States, Caribbean Islands and South America. Otherwise, southwest-southeast splits in autumn migratory movements are less obvious within North America than in Eurasia, although they occur in many northern forest species which tend to avoid the central prairies (Chapter 18). Nevertheless, because most of South America lies east of North America, most intercontinental migration occurs on a northwest-southeast axis, at least for part of the route.

In general, barriers may have at least three main consequences for bird migration: (1) they may stop onward movement altogether; (2) they may lead to genetically controlled or optional detours, where at one or both migration seasons the crossing of the barrier is circumvented, thus lessening the risks, and providing opportunities

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