Longitudinal patterns

Typically, birds that breed furthest west in the breeding range tend to winter furthest west in the non-breeding range, and those that breed furthest east also winter furthest east (Figure 23.1). Allowing for the uneven distribution of land masses, this holds in both the Old and New Worlds, reflecting the more or less parallel migrations of populations. Such longitudinal patterns have been revealed by ring recoveries from most groups of birds (Lincoln 1935b, Moreau 1972, Newton 1972, Holmes & Sherry 1992, Hoffman et al. 2002), and in some groups they have also been broadly confirmed by isotope or DNA analyses, as described in Chapter 2 (for Dunlin Calidris alpina see Wenink et al. 1996, Wennerberg 2001; for Swainson's Thrush Catharus ustulatus see Ruegg & Smith 2002; for Wilson's Warbler Wilsonia pusilla see Kimura et al. 2002).

Based mainly on findings from waterfowl, Lincoln (1935b) proposed that migratory birds in North America followed four main flyways, west to east: the Pacific, Central, Mississippi and Atlantic Flyways. This division soon came to form the political basis for waterfowl management and hunting throughout the continent. For the most part, it reflects biological reality, because of the more or less parallel migrations. The concept was further developed by Bellrose (1968) who used radar surveillance and ringing records of migratory waterfowl to describe the presence of migration corridors within the flyways. He defined a corridor as a narrow strip of airspace used by waterfowl as they migrated between their breeding and wintering grounds. In some places the corridors were little more than 16 km wide, in others more than 200 km. Such more-or-less parallel migrations need not necessarily involve competition, however, for they could be explained by the birds from different segments of the breeding range taking the shortest routes to their wintering areas at lower latitudes. Other factors being equal, any birds that deviate from the parallel pattern, and cross the routes of other populations, are likely to lengthen their journeys, and thus have greater migration costs.

Figure 23.1 Examples of parallel migration among west Palaearctic breeding birds, from ring recoveries. (a) Chaffinches Fringilla coelebs ringed at the Courland Spit, Russia (filled square, filled dots) and at the Col de Bretolet (open square, open dots). From Bairlein (1998).

The migration corridors of some species are narrow over almost the whole route, as in some cranes, geese and shorebirds that travel between traditional widely spaced stopover and wintering sites. For example, Whooping Cranes Grus americana from a small breeding area in northern Canada migrate along a narrow corridor direct to their only wintering site at Aransas, on the Texas coast, then back along the same route (Figure 23.2). Different populations of geese remain separated year-round as they move from different breeding localities, along different flyways to their own traditional wintering areas. Examples include Barnacle Geese Branta leucopsis and Pink-footed Geese Anser brachyrhynchus in both of which birds from three separate breeding areas have distinct migration and wintering areas (Figure 23.3). Breeding and wintering localities may increase in number as a population expands, but localities that lie off the migration routes are seldom, if ever, visited by such species. Similarly, many shorebirds tend to

Figure 23.1 (Continued) (b) Northern House Martins Delichon urbica from Europe to Africa. From Hill (1997).

travel along coastlines which provide potential stopping sites, if needed. This situation of narrow, separated routes is no different in principle from the parallel but broad-front migration of most widespread species, except that the different breeding or wintering areas are localised and well separated.

Whatever their history, the different populations (or subspecies) of some widespread species show almost total separation year-round, whether in breeding, migration or wintering areas. An example is the Red Knot Calidris canutus, which breeds at high arctic latitudes throughout the world, and performs long and complex migrations to reach wintering areas that extend from northern temperate to southern temperate regions. The population associated with each breeding area migrates to a discrete wintering area (Figure 23.4). In other shorebird species, different populations have separate breeding and wintering areas, but come together

Figure 23.2 Autumn and spring migration corridors of Whooping Cranes Grus americana, as defined by radio-tracking, between their sole breeding area in Canada and their sole wintering area at Aransas, on the Texas coast. The map is a Mercator projection. Based on Kuyt (1992) and Alerstam (1996).

at migration times, often mingling on their shared staging sites. For example, in autumn and spring, Ruddy Turnstones Arenaria interpres from two populations occur in Britain, one from northeast Canada and Greenland and the other from the Baltic region. The former stay all winter, while the latter pass through to winter in West Africa.

Many other examples of partially allopatric winter distributions are found among waders in Britain, with birds from breeding areas to the northwest (Iceland, Greenland and Canada) wintering mainly in Ireland and western Britain, and birds from breeding areas to the northeast (Eurasia) wintering mainly in eastern Britain, the Common Snipe Gallinago gallinago providing a good example. Similarly, the Black-tailed Godwits Limosa limosa that breed in Iceland winter in the British Isles, while those that breed in continental Europe winter in Europe and western Africa. Although not strictly parallel migrations, they still result in birds maintaining the

Figure 23.3 Breeding areas, migration routes and wintering areas of three populations of Barnacle Geese Branta leucopsis and two populations of Pink-footed Geese Anser brachyrhynchus. Black shading depicts the separate breeding and wintering areas, grey the migration routes, and white spots the main stopover sites. From Madsen et al. (1999).

Figure 23.3 Breeding areas, migration routes and wintering areas of three populations of Barnacle Geese Branta leucopsis and two populations of Pink-footed Geese Anser brachyrhynchus. Black shading depicts the separate breeding and wintering areas, grey the migration routes, and white spots the main stopover sites. From Madsen et al. (1999).

Figure 23.4 Year-round separation of different subspecies of Red Knots Calidris canutus. These birds breed at high arctic latitudes around the world, different subspecies in different regions. Vertical stripes - wintering areas; horizontal stripes - stopover areas used only during southward and northward migration; cross-hatched - areas used for both stopover and wintering; shaded corridors - proven migration routes; broken shaded corridors - possible migration routes on which more information is needed. The population islandica breeds in eastern Canada-Greenland, and migrates through Iceland to winter in western Europe. The Siberian canutus stages in the Wadden Sea, across the English Channel from islandica birds, but migrates on to winter on the western coasts of Africa. The subspecies rogersi breeds in eastern Siberia and winters in southeast Australia and New Zealand. Other birds (considered piersmai) breed on the New Siberian islands and winter in northwest Australia. The race roselaari breeds in far eastern Siberia and northern Alaska and probably overwinters in Florida-Texas, and possibly the northern coast of South America. The race rufa breeds in the central Canadian arctic and winters on the Atlantic coast of Argentina. No Red Knots are known to winter around the Indian Ocean, except for vagrants. This example, along with many other shorebirds, geese and others, indicates how populations maintain their distinctive distributions year-round (Piersma & Davidson 1992).

same relative west-east sequence in both breeding and wintering areas (Wernham et al. 2002).

In parallel migration, then, allowing for geography, species maintain the same west-east distribution with respect to one another year-round. In contrast to this pattern is fan (or funnel) migration, in which birds from a small part of the breeding

Figure 23.5 Fan migration as illustrated by Peregrine Falcons Falco peregrinus trapped and attached with radio-transmitters during winter on the Gulf Coast of Mexico (north of Tampico) and subsequently tracked to breeding areas across the Arctic. Lines connect the winter trapping site with the subsequent breeding sites but do not necessarily depict the routes flown. From details in McGrady et al. (2002).

Figure 23.5 Fan migration as illustrated by Peregrine Falcons Falco peregrinus trapped and attached with radio-transmitters during winter on the Gulf Coast of Mexico (north of Tampico) and subsequently tracked to breeding areas across the Arctic. Lines connect the winter trapping site with the subsequent breeding sites but do not necessarily depict the routes flown. From details in McGrady et al. (2002).

range can spread out to occur across a wide part of the wintering range or vice versa (Figure 23.5). This results in the individuals in particular wintering localities being drawn from a wide longitudinal spread in the breeding range or, conversely, individuals in particular breeding localities being drawn from a wide longitudinal spread in the wintering range. There is no clear dividing line between parallel and fan migration, but rather different species reveal a continuum of variation between these extremes, depending partly on geography. In other words, species show different degrees of year-round longitudinal segregation. Extreme mixing would be expected to be rare among most kinds of birds, because it would entail some individuals migrating much further than they need. However, many raptors and other soaring species become funnelled on migration through points with narrow land bridges (such as Panama) or short sea-crossings (such as Gibraltar) (Chapter 7). Their various migration routes are shaped like an hourglass, constrained in the middle.

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