Rustic Bunting Emberiza rustica Little Bunting Emberiza pusilla Yellow-breasted Bunting Emberiza aureola including India

Note: In addition to the species listed above, Richard's Pipit Anthus novaeseelandiae breeds in the eastern Palaearctic, Africa and Australasia, but migrates in small numbers through western Europe, presumably en route to Africa.

breed in western Europe, such as Marsh Warbler Acrocephalus palustris and Lesser Whitethroat Sylvia curruca, also migrate eastwards before turning south into Africa. Why do they not fly directly southwest through Spain into Africa like many other species do? Perhaps there is an adaptive explanation for this roundabout migration route, but another possibility is that these species survived the last glaciation only in southeast Europe-eastern Africa, and in post-glacial times spread to occupy for breeding first the eastern then western side of the European continent. Their current migration routes could again trace the ancestral routes of colonisation, with the western birds having failed to evolve a new and more direct route through Iberia into western Africa. Such a change in west European birds would have needed a marked change in migratory direction, together with a longer trans-desert flight on the west side of Africa than is necessary on the east side.

The opposite situation is shown by the European Pied Flycatcher Ficedula hypo-leuca in which practically the whole Palaearctic population from as far as 93°E (north of Mongolia) apparently passes through Iberia in autumn en route to West African wintering areas. It contrasts with the closely related Collared Flycatcher F. albicollis in which the whole population travels through Italy and southeast Europe into African wintering areas that lie to the east and south of those used by the European Pied Flycatcher. One of the most curious migrations is shown by the Aquatic Warbler Acrocephalus paludicola which breeds in eastern Europe but migrates first west to the North Sea coasts, including Britain, before turning south to Africa. It is possible that this is the optimal route to its African wintering areas, but another possibility is that this species colonised eastern Europe from the west, then died out as a breeder in the west but retained its original migration route.

Many other examples of curious routes to distant wintering areas can be found among Eurasian birds, and in many different families. But such long and indirect migration routes are not restricted to the Old World. In North America, some species breed across the northern parts of the continent but concentrate to winter at lower latitudes entirely in the west or entirely in the east. About 33 species of the eastern forests have spread westward, north of the prairies, in the boreal forests, yet migrate through the east of the continent en route to Neotropical wintering areas. The Bobolink Dolichonyx oryzivorus has spread westward in the last 200 years. This species winters in southern South America, and the western birds start their migration by flying east, instead of taking a more direct route (Lincoln 1935a). It is easy to imagine how a new route could evolve, step by step in whatever direction range expansion occurred, with each successive extension adding to the pre-existing route.

The underlying assumption here is that, in the genetic control of migration, progressive minor extensions or modifications to an existing programme are easier to achieve than are abrupt big-step changes. Such 'evolutionary inertia' may account for some of the variants we see in existing migration routes, some species having achieved the change to markedly different routes, while others have achieved only extensions to former routes.

For many years, the view that some migration routes retraced ancestral routes of spread was no more than plausible speculation, but recent DNA analyses have added support. Parts of the DNA in organisms are thought to have no effects on the fitness of the individual but to change at fairly constant rates over time, owing to mutations which alter the sequence of individual nucleotides along the length of the molecule. By comparing this sequence in equivalent pieces of DNA (usually mitochondrial DNA) from individuals from different parts of the breeding range, it is sometimes possible to separate populations, and even to work out the order in which different populations arose, and hence the historical pattern of range expansion. For example, Swainson's Thrush Catharus ustulatus is separated into two genetically distinct populations, one of which occupies the west coast region of North America north into Canada, and the other the rest of the continental range from Alaska to Newfoundland. The two populations meet and interbreed around the British Columbian-Alaskan border. To judge from molecular dating, the two populations diverged during the last glaciation, when they were probably confined to separate southwestern and southeastern refuges. They now have migration routes that could retrace their likely routes of spread from these two areas (Ruegg & Smith 2002). Ring recoveries and genetic data from this species show nearly complete segregation of migratory routes and wintering areas. Western coastal populations migrate along the Pacific coast to Central America and Mexico, whereas continental populations migrate eastward and then southward to Panama and South America. The circuitous spring migration of the eastern continental population of this species, northward then westward, mirrors the expansion route of the boreal forest (where the species breeds), following the last glacial retreat. Other North American species with similar distributions and indirect migration routes include the Hermit Thrush Catharus guttatus and Gray-cheeked Thrush C. minimus (Brewer et al. 2000). Western and eastern types of other North American birds also differ in mitochondrial DNA (e.g. Yellow Warbler Dendroica petechia, Milot et al. 2000; Fox Sparrow Passerella iliaca Zink 1994), as do some European birds (e.g. Great Reed Warbler Acrocephalus arundinaceus, Bensch & Hasselquist 1999), suggesting that they diverged in separate western and eastern refuges. Some species may still breed in these refuges, as well as in more northern areas colonised since then, but other species may have abandoned their glacial refuges as breeding areas, now using them only as wintering or passage areas.

Similarly, two populations of Willow Warblers Phylloscopus trochilus breed in Sweden, the subspecies trochilus in the south and the subspecies acredula in the north. The two populations meet at about 62°N. They differ morphologically, and show different migration routes to different wintering areas; trochilus moving southwest into West Africa and acredula southeast into eastern and southern Africa (Bensch et al. 1999, 2002, Chamberlain et al. 2000). They can also be distinguished by their DNA or by the isotope ratios in their feathers, reflecting their different wintering areas where moult occurs (Figure 22.2). Their contact zone, at about 62°N and spanning >350 km, is paralleled in other species that breed in Scandinavia, both birds and other animals. Such examples are most plausibly explained by the postglacial colonisation of Scandinavia along two routes, one from the southwest and the other from the east, over the northern end of the Baltic.

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