All else being equal, energy costs would normally favour birds taking the shortest, most direct routes between their breeding and wintering areas, which would be as close together as possible. Yet many birds take long roundabout routes between one area and the other, or use breeding and wintering areas that are separated by extraordinarily long distances, while other apparently suitable areas much closer remain unused. The problem is to understand why. Some roundabout routes might be explained on grounds of safety, if the shortest route is more risky (say with unfavourable winds or a lengthy sea-crossing), or on grounds of competition avoidance (if closer areas are already occupied by other populations of the same or similar species). However, these explanations seem unable to account for all such patterns. Almost certainly, the explanation of some long routes rests in past colonisation patterns.
A striking example is provided by the Northern Wheatear Oenanthe oenan-the, which from Eurasia has colonised Greenland and northeast Canada in the west and Alaska in the east, yet birds from all these breeding areas continue to migrate to Africa (Figure 22.1). The Greenland-Canadian birds cross the Atlantic, one of the longest sea-crossings undertaken by a passerine, while the Alaskan birds cross the Bering Sea and travel via Siberia and then the Middle East, covering a distance of 15 000 km twice each year. If Wheatears from the two ends of the breeding range were to migrate instead to South America or to Southeast Asia respectively, where no Wheatears currently winter, they could halve their migration distances. Perhaps Africa is the only continent with suitable wintering habitat for Wheatears, but a more likely explanation is that the outlying populations have failed to evolve a new route. The problem with a switch from Africa to South America is that there are now no suitable intermediate routes, so no opportunity for gradual change. Thus Northern Wheatears from northeast Canada, for example, would need a big change in direction to end up in South America rather than in Africa and any progressive change by intermediate directions (from east-southeast to south) would take them to the South Atlantic. Similarly, any Northern Wheatears from Alaska would need a big directional change to reach South America, while small stepwise changes would land them in the Pacific.
It is not just the Northern Wheatear that behaves in this way. Several species that have colonised Alaska from the east end of Eurasia (such as the Willow Warbler Phylloscopus trochilus and Yellow Wagtail Motacilla flava) continue to winter in the Old World, as do other species (such as Common Ringed Plover Charadrius hiaticula and Red Knot (race Calidris canutus islandica) that have colonised Greenland and northeastern Canada from Eurasia. Likewise, several other species (such as Grey-cheeked Thrush Catharus minimus and Pectoral Sandpiper
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