Loop Migrations

This term is applied to the many populations that take markedly different routes on their outward and return journeys. Loop migrations are widespread among birds, having been described for passerines, shorebirds, raptors, seabirds and others from many different parts of the world (Figures 22.5 and 22.6). Some European species travel into Africa via Iberia, but return by a more central route mainly through Italy (e.g. Garden Warbler Sylvia borin, European Pied Flycatcher Ficedula hypo-leuca), the whole migration following an anticlockwise loop. Yet other species travel south over the eastern Mediterranean, and back further east mainly via Arabia on another anticlockwise loop (e.g. Red-backed Shrike Lanius collurio). In loop migrations, most individuals in a population take different routes in spring and autumn, but not necessarily all of them. They cause big seasonal differences in the numbers of birds seen at particular stopover sites, as some sites that are favoured in autumn are almost deserted in spring, and vice versa. In association with the different

Figure 22.5 Some well-documented loop migrations in different regions.

A. Western North America, shown by various waterfowl, shorebirds and others, notably Brent Geese Branta bernicla and Western Sandpiper Calidris mauri.

B. Eastern North America, shown by various passerines, shorebirds and others, notably Blackpoll Warbler Dendroica striata. C. Western Europe-North Africa, shown by various passerines and others, notably Pied Flycatcher Ficedula hypoleuca (see also Figure 22.7). D. Middle East-North Africa, shown by various passerines and others, notably Red-backed Shrike Lanius collurio.

Figure 22.5 Some well-documented loop migrations in different regions.

A. Western North America, shown by various waterfowl, shorebirds and others, notably Brent Geese Branta bernicla and Western Sandpiper Calidris mauri.

B. Eastern North America, shown by various passerines, shorebirds and others, notably Blackpoll Warbler Dendroica striata. C. Western Europe-North Africa, shown by various passerines and others, notably Pied Flycatcher Ficedula hypoleuca (see also Figure 22.7). D. Middle East-North Africa, shown by various passerines and others, notably Red-backed Shrike Lanius collurio.

routes, patterns of migratory fuelling may also differ between autumn and spring, depending on the numbers and spacing of stopping sites (Chapter 5).

Loop migrations can usually be explained in terms of the conditions encountered en route. In eastern North America, as well as in Europe, prevailing winds north of 35-40°N blow from west to east, whereas south of 30-35°N they blow from east to west. Many species in eastern North America make their southward journey to the east of their northward journey. This is most strikingly apparent in those species that fly over the Atlantic between northeastern North America and northeastern South America in autumn, but return in spring over the land route further west. Prevailing winds favour their autumn journey over the Atlantic, but would be inimical there in spring (Chapter 4). In other species, feeding conditions seem to be important. For example, in western North America, several hummingbird species migrate up the Pacific coast in spring, taking advantage of the spring flowers on low ground, and down the Rocky Mountains in autumn when flowers are more plentiful on high ground (Phillips 1975). Male Hammond's Flycatchers Empidonax hammondii move

Figure 22.6 'Loop migration' of European Pied Flycatchers Ficedula hypoleuca ringed near Brunswick in northern Germany (open square). Black dots: recoveries during autumn migration (n = 71); circles: recoveries during spring migration (n = 40); arrows: main migration directions in autumn and spring. Autumn migration follows a fairly narrow westerly route, whereas spring migration follows a much broader and more easterly route. The recoveries suggest that birds stop mainly in southwest Europe on their autumn journey to west tropical Africa and mainly in North Africa on their return spring journey. From Bairlein (2001), based on Winkel & Frantzen (1991).

Figure 22.6 'Loop migration' of European Pied Flycatchers Ficedula hypoleuca ringed near Brunswick in northern Germany (open square). Black dots: recoveries during autumn migration (n = 71); circles: recoveries during spring migration (n = 40); arrows: main migration directions in autumn and spring. Autumn migration follows a fairly narrow westerly route, whereas spring migration follows a much broader and more easterly route. The recoveries suggest that birds stop mainly in southwest Europe on their autumn journey to west tropical Africa and mainly in North Africa on their return spring journey. From Bairlein (2001), based on Winkel & Frantzen (1991).

north up the Pacific coast of North America, while females, which migrate later in spring when conditions have improved, take a more direct inland route (Johnson 1965). Another impressive example of loop migration dependent on ground conditions in central Asia was described in Chapter 6 (see Figure 6.4).

Evidence that both the outward and return routes are under genetic influence comes from the directional preferences shown by birds in captivity, which are not simply reversed between autumn and spring. For instance, captive Garden Warblers Sylvia borin from central Europe, tested in orientation cages throughout their periods of migratory restlessness, initially showed a southwest heading in autumn, but after some days they switched to southeast (Chapter 20). This change fits with their migration southwest via Iberia and then southeast into tropical Africa. In spring, by contrast, captive birds headed directly north throughout, which fits with their normal return through Italy (Gwinner & Wiltschko 1978, 1980). In many populations that perform loop migrations, one route might be considered as the ancestral route, perhaps following the path of colonisation, and the other the derived route, evolved to shorten the distance or to avoid adverse conditions. Their importance is in showing that selection pressures on the outward journey can act independently of those on the return journey. The one route only partly predetermines the other. Loop migrations require no special explanation, then, but they do require more complex directional inheritance than direct back and forth journeys by the same route. This does not, of course, exclude the possibility that some individuals respond facultatively to conditions at the time. Habitat conditions may influence the numbers seen on the ground, and in many areas birds break their journeys in greater numbers, or for longer periods, in years when food is abundant than in other years, as is obvious to any experienced bird-watcher.

0 0

Post a comment