Other more familiar types of movements are analogous to overshoots, except that the movement occurs in response to external conditions outside the normal migration season. At the start of exceptionally severe winter weather, birds are sometimes driven to lower latitudes than usual, with occasional individuals reaching much further than the rest, to be classed as vagrants. Such movements are common among various waterfowl and waders that are affected by freezing waters and soils, or ground-feeding thrushes and seed-eaters whose food is covered by snow too deep to penetrate (Chapter 16). Similarly, during hard winters in mountain areas, birds may be driven to lower altitudes than usual, appearing as vagrants outside their regular winter range. European examples include the Alpine Accentor Prunella collaris and Wallcreeper Tichodroma muraria, and North American examples include Clark's Nutcracker Nucifraga columbiana and various rosy finches Leucosticte. Irruptive species also occasionally appear well beyond their usual invasion areas.
The occurrence of severe droughts in the desert and semi-desert regions of southern Asia sometimes results in the appearance of arid land species in western Europe. Examples include the Pallas's Sandgrouse Syrrhaptes paradoxus and Rose-coloured Starling Sturnus roseus, and also Ruddy Shelduck Tadorna ferruginea and Glossy Ibis Plegadis falcinellus, which often appear in the same summers as one another, as in 1886, 1892 and 1994 (Cottridge & Vinicombe 1996). Irruptions of Pallas's Sandgrouse were noted in 12 years between 1859 and 1909, the largest in 1863 and 1888, but the species has appeared much less often and in only small numbers subsequently (Chapter 16).
The distributions of many marine birds are closely tied to particular current systems, whose distribution periodically changes from the normal, as during El Niño events. When this happens, the seabirds associated with those currents also change distribution, disappearing from their usual areas of occurrence and appearing elsewhere, hundreds of kilometres distant. This results in mass distributional changes, together with the appearance of vagrants in more distant areas than usual.
Yet another mechanism that occasionally brings vagrants to western Europe occurs particularly in waterfowl when an individual of one species attaches itself to a flock of another, and ends up in the wrong breeding or wintering area. For example, a Ferruginous Duck Aythya nyroca, which would normally winter in the eastern Mediterranean region, may attach itself to a flock of Common Pochards A. ferina in late summer and head off with them towards the west. Similarly, in western Europe, Falcated Teal Anas falcata occasionally turn up with Eurasian Wigeon A. penelope, Lesser White-fronted Geese Anser erythropus with Greater White-fronts A. albifrons, and Red-breasted Geese Branta ruficollis with Brent Geese B. bernicla. These are all closely related species which either breed in the same area as one another, or cross routes on migration, giving the potential for intermixing to occur. Similarly, Common Cranes Grus grus and Sandhill Cranes Grus canaden-sis both breed in parts of eastern Siberia, and Common Cranes (which normally winter in the Old World) have occasionally been seen among Sandhill Cranes in North American wintering areas.
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