In western Europe, the usual migration direction of most irruptive species is southwest, a direction in which birds could usually be expected to find suitable habitat and food supplies, especially in montane conifer forests. Birds from further east, however, tend to have a stronger westward component in their movements, which brings them to western Europe, either within the boreal forest itself, or south of it into the temperate region. They thereby largely avoid the dry steppe and desert lands of Asia in which their survival chances would presumably be low. This westerly movement from Siberia is especially marked in Nutcrackers, Common Crossbills, Two-barred Crossbills, Siberian Nuthatches and Pine Grosbeaks, which leads the invasions of some of these species to be more obvious in Fennoscandia than further south.
Such patterns led Svardson (1957) to suggest that some boreal birds performed regular pendulum movements, moving first west then east across the boreal forest of Eurasia, a view for which there is only limited evidence beyond the eastward return after occasional westward irruptions. However, some ringed Redpolls Carduelis flammea showed easterly autumn movements within the boreal zone. Of two ringed in northern Norway in August 1977, one was reported 3091 km due east near Novosibirsk two months later on 20 October 1977, and another was reported
2753 km east-southeast near Chaklovo on 12 March 1978. In the 1965 irruption, some birds ringed in Finland reached the edge of the Altai in central Asia, over distances up to 3573 km.
In North America, too, there is a strong east-west component in the movements of some irruptive species. Evening Grosbeaks Hesperiphona vespertina, Common Redpolls Carduelis flammea and Pine Siskins C. pinus from the northwest of the continent tend to move east-southeast in autumn, as mentioned earlier, while Purple Finches Carpodacus purpureus move first east-southeast to the Great Lakes region, then veer south towards Texas and Louisiana (Houston & Houston 1998, Brewer et al. 2000). Similarly, observations suggested a large movement of White-winged Crossbills Loxia leucoptera from Alaska and Yukon across the boreal forest to Ontario and Quebec in May-June 1984, and a return movement in May-June of the following year (Benkman 1987). In addition, the small-billed Red Crossbill L. c. sitchensis may regularly make movements between the Pacific Northwest, including Alaska, and the Great Lakes region (Benkman 1987). By these movements, all species avoid the desert and prairie areas of the mid-west, and remain largely in forested areas that provide their food.
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