Latitudinal patterns

The latitudinal distribution of populations in the non-breeding season is more complicated than the longitudinal. At least three main patterns are found within species (Figure 23.6). In the first pattern, of chain migration, more northern (usually larger) birds may replace others as they move south, so that the same areas are occupied year-round, but by one population in summer and another in winter. An example of chain migration within a species is provided by the Common Redshank Tringa totanus in which Icelandic birds move to Scotland and the North Sea area, while local breeding birds move further south (Summers et al. 1988). Another example is provided by the Purple Sandpiper Calidris maritima, in which birds from four populations move to the nearest ice-free coast for the winter, replacing birds from two other populations that move further south (Summers 1994). Other examples are provided by the Chiffchaff Phylloscopus collybita in Iberia (Catry et al. 2005), the European Goldfinch Carduelis carduelis, Linnet Carduelis can-nabina, White Wagtail Motacilla alba, Eurasian Sparrowhawk Accipiter nisus, Tufted Duck Aythya fuligula and Common (Mew) Gull Larus canus over wider parts of Europe (Salomonsen 1955, Kilpi & Saurola 1985, Siriwardena & Wernham 2002), and by the American Coot Fulica americana, Sharp-shinned Hawk Accipiter stria-tus, Hermit Thrush Catharus guttatus and American Redstart Setophago ruticilla in North America, the two latter patterns established by isotope analyses (Phillips 1951, Ryder 1963, Smith et al. 2003, Norris et al. 2006).

The second latitudinal pattern of leapfrog migration is found commonly within various species of passerines, raptors, waders, waterfowl, gulls and other seabirds (Salomonsen 1955, Moreau 1972, Alerstam & Hogstedt 1980, Kilpi & Saurola 1985, Bourne 1986, Wallin et al. 1987, Boland 1990, Wood 1992, Siriwardena & Wernham

Leapfrog Chain Telescopic migration migration migration

Leapfrog Chain Telescopic migration migration migration

Figure 23.6 Three common patterns of latitudinal replacements in the migrations of various bird species. Partly from Salomonsen (1955). Chain migration in which populations occur in winter in the same latitudinal sequence as on their breeding areas. Leapfrog migration in which populations occur in winter in reverse latitudinal sequence as on their breeding areas. Telescopic migration, in which populations from different breeding areas occur together in the same wintering area.

Figure 23.6 Three common patterns of latitudinal replacements in the migrations of various bird species. Partly from Salomonsen (1955). Chain migration in which populations occur in winter in the same latitudinal sequence as on their breeding areas. Leapfrog migration in which populations occur in winter in reverse latitudinal sequence as on their breeding areas. Telescopic migration, in which populations from different breeding areas occur together in the same wintering area.

2002). Well-known examples are provided by the Fox Sparrow Passerella iliaca west of the Rockies in North America (Swarth 1920, Bell 1997) and by the Common Ringed Plover Charadrius hiaticula in Europe (Figure 23.7; Salomonsen 1955). In the last species, the British population is mainly resident, while the southern Scandinavian birds winter in southwest Europe and the northernmost Scandinavian-Siberian birds winter largely in Africa. Icelandic birds also migrate to Africa, thereby leapfrogging the European ones. Among seabirds, Common Terns Sterna hirundo from Britain winter around the western bulge of Africa mainly north of the equator, while those from Scandinavia winter mainly south of the equator on the western coast (Wernham et al. 2002). Cory's Shearwaters Calonectris diomedea from the Cape Verde Islands winter mainly in latitudes to 24°S, whereas those from more northern colonies winter south to 48°S (Bourne 1986). In all these examples, leapfrog patterns were established by ringing, but in recent years such patterns have also emerged through isotope analyses (for Wilson's Warbler Wilsonia pusilla see Clegg et al. 2003).

Leapfrog migrations could have arisen in post-glacial times through competition, if populations spreading progressively further north to breed had to migrate ever further south to avoid competing with other populations and find unoccupied wintering areas (Chapter 22). Without such competition, the most northern breeding populations would be expected to shorten their journeys. In some species that show leapfrog migration, the northern breeding birds are smaller than the southern ones (see later). As in closely related species pairs, they follow

Figure 23.7 Leapfrog migration in: (a) Fox Sparrow Passerella iliaca; and (b) Common Ringed Plover Charadrius hiaticula. From Swarth (1920) and Salomonsen (1955).

Bergman's Rule that high-latitude forms are larger, but the trend holds for wintering rather than for breeding distributions.

In the third latitudinal pattern, populations that breed over a wide span of latitude become telescoped in the non-breeding season into a narrower span, so that birds from different breeding areas intermix in winter. Populations that are allopatric in summer thus become sympatric (or 'synheimic') in winter. This pattern is illustrated by Common Grackles Quiscalus quiscula and Common Starlings Sturnus vulgaris in eastern North America (Dolbeer 1982; Figure 23.8), Rosy Finches Leucosticte arctoa, L. tephrocotis tephrocotis and L. a. littoralis in western North America (King & Wales 1964), several races of Yellow Wagtails Motacilla flava in parts of Africa (Salomonsen 1955, Curry-Lindahl 1958, Cramp 1988), and by various races of some Nearctic-Neotropical migrant species in parts of Mexico (Ramos & Warner 1980). Mixing among birds from widely different breeding areas also occurs in winter in various seabirds, including Black-legged Kittiwake Rissa tridactyla, Brunnich's Guillemot (Thick-billed Murre) Uria lomvia, Northern Fulmar Fulmarus glacialis, Sandwich Tern Sterna sandvicensis and others (Salomonsen 1955). In many other species, northern migratory races winter within the range of more southern resident ones, as in Richard's Pipit Anthus novaeseelandiae (Salomonsen 1955, Cramp 1988).

Telescopic migration also applies to migration from wintering to breeding areas, as populations wintering over a wide span of latitude become concentrated for breeding within a narrower span. Many examples occur among shorebird species which occupy coastlines over much of the world in winter, but withdraw to breed in a relatively narrow span of arctic tundra.

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