Further comments on the role of glacial changes

Recent estimates suggest that more than 20 glaciations occurred in the last 2.5 million years, together occupying about 90% of this period (Newton 2003). In each one, ice sheets spread from the poles to much lower latitudes than today, obliterating vegetation or pushing it southward, along with its animal inhabitants. During the peak of the glaciations, most European bird species were confined

Figure 22.2 Recoveries of Willow Warblers Phylloscopus trochilus ringed in Sweden. Those from birds ringed in southern Sweden are shown as filled circles and those from northern Sweden as squares. The breeding distribution in Sweden shows the approximate location of the hybrid zone (shaded grey) between two subspecies (P. t. acredula in the north and P. t. trochilus in the south), where there is a migratory divide. The diagram shows the 15N signature in birds from southern Sweden, the hybrid zone, and northern Sweden. These data reflect the location of the winter moult in Africa, and hence the differing wintering areas of the two populations. The hybrid zone marks an area where ice during the last glaciation was unusually thick, so that land to the north and south cleared much earlier, allowing colonisation by plants and animals. When the ice went completely, organisms from the south and north could meet, producing hybrid zones, and in the case of the Willow Warbler, a migratory divide. Based on Chamberlain et al. (2000).

Figure 22.2 Recoveries of Willow Warblers Phylloscopus trochilus ringed in Sweden. Those from birds ringed in southern Sweden are shown as filled circles and those from northern Sweden as squares. The breeding distribution in Sweden shows the approximate location of the hybrid zone (shaded grey) between two subspecies (P. t. acredula in the north and P. t. trochilus in the south), where there is a migratory divide. The diagram shows the 15N signature in birds from southern Sweden, the hybrid zone, and northern Sweden. These data reflect the location of the winter moult in Africa, and hence the differing wintering areas of the two populations. The hybrid zone marks an area where ice during the last glaciation was unusually thick, so that land to the north and south cleared much earlier, allowing colonisation by plants and animals. When the ice went completely, organisms from the south and north could meet, producing hybrid zones, and in the case of the Willow Warbler, a migratory divide. Based on Chamberlain et al. (2000).

to one or more of the three southern peninsulas of Iberia, Italy and the Balkans, although the two latter were joined across the top of the Adriatic Sea. Other glacial refuges existed in North Africa and on Mediterranean Islands, as well as to the east of the Mediterranean. The islands were larger than today, and in some cases joined together, owing to lower sea levels. During each interglacial, the ice retreated, allowing vegetation to spread back again, providing new habitat for birds and other animals. Both plants and animals spread to higher latitudes from their glacial refuges. During each period of spread, migration systems would have had to adjust. Hence, many existing migration routes, assumed to follow ancestral routes of spread, are likely to pass through the breeding areas occupied in glacial times. Depending on species and area, some such refuges may still be used for breeding and wintering, but others may not. Some species are thought to have survived the last glaciation in only one refuge system, and others in more than one. This is a plausible hypothesis for why in some species the whole migration filters through one region, whereas in other species migrations separate into different streams towards different regions.

In species occupying more than one glacial refuge, the separate populations sometimes differentiated, giving separate species or subspecies, which subsequently spread out from their refuges to form overlap or hybrid zones where they met (for review see Newton 2003). Many such 'suture' zones run roughly north-south through central Europe, marking the overlap between such taxa as Nightingale Luscinia megarhynchos and Thrush Nightingale L. luscinia, and Carrion Crow Corvus corone and Hooded Crow C. c. cornix. The precise position of the suture zone varies from species to species, possibly reflecting differential rates of spread from separate glacial refuges. Other suture zones occur through western North America, marking the overlap zone between such species as Lazuli Bunting Passerina amoena and Indigo Bunting P. cyanea, and Bullock's Oriole I. g. bullockii and Baltimore Oriole I. g. galbula. These and other populations, on either side of these suture zones, whether classed as species or subspecies, apparently retained their ancestral migration routes, giving one plausible explanation for the existence of 'migratory divides', on either side of which the population follows a different route to different winter quarters (see later). Such divides are also apparent in taxonomically undifferenti-ated populations, such as the White Stork Ciconia ciconia and Blackcap Sylvia atrica-pilla in Europe (although western and eastern populations of such species may be separable through DNA analyses). This is the case in the Great Bustard Otis tarda, in which Spanish birds are separable on both mitochondrial and nuclear DNA from Hungarian ones (Pitra et al. 2000) but, with bustards having disappeared from the intervening terrain, it is not clear where the divide may once have been.

Knowledge of glacial history thus provides a plausible hypothesis of why some species, which now breed across a continental land mass, migrate to one end of that land mass while others migrate to the other end or to both ends. During evolution, selection would be expected to shorten and straighten migration routes, in order to reduce the energy costs, but in some species this process seems yet to have occurred to only a limited extent.

Based on present-day examples, many birds may have crossed the ice sheets during the glacial periods in order to breed on the restricted vegetated areas that persisted north of the ice. This idea is supported by the patterns of subspeciation seen in waders and other high-arctic breeding species today, in many of which different subspecies are centred on areas known to have remained ice-free in summer throughout the glacial periods (Ploeger 1968, Newton 2003). In this case, birds could have nested at some locations in the far north throughout the Pleistocene glaciations, as well as in the interglacials. During the glaciations, however, they would have to have crossed hundreds or thousands of kilometres of ice between their breeding and wintering areas, longer distances than are covered today by those species that cross the Greenland ice sheet.

While many populations of birds owe their present genetic structuring to fragmentation of their populations into 'refuges' during the climatic extremes of the glacial or interglacial periods, the subsequent maintenance of this structure is due largely to behavioural philopatry, in which individual birds return from migration year after year to breed near their natal areas. The Dunlin Calidris alpina provides a good example. This species now breeds widely in northern Eurasia and North America, but five main genetic lineages have been recognised, each centred on a different breeding area (Wenink et al. 1993, 1996). All these lineages appear to have differentiated in isolation in different refugia within the latter half of the Pleistocene, and to have maintained their differences as a result of strong philo-patry; the birds from each area have expanded to colonise new breeding areas and, to some extent, mixed with other populations, but the ancient genetic subdivisions are still apparent today in the mitochondrial DNA. Birds sampled on migration can therefore be allocated to one of these five lineages, and hence to a particular part of the breeding range.

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