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Another problem with the reversed migration hypothesis is that the vagrancy shadow produced by 180° reversal of orientation for Siberian migrants (which normally migrate to Southeast Asia) falls in Britain and other parts of northwest Europe where there are lots of observers (Gilroy & Lees 2003). On almost all other compass points to which a Siberian vagrant might head, observer coverage is much less thorough. However, observer bias could not account for the concentration of eastern North American warblers in mid-coastal California, for areas to the south and north are also well watched, or for the occurrence of western American Dendroica warblers on the northeast coast, including the Nova Scotia Islands (McLaren 1981). Many other regular autumn migrants from Siberia occur occasionally in western Europe, even though they are not within their migration shadow. Examples include the Pied Wheatear Oenanthe plechanka, River Warbler Locustella fluviatilis, Eastern Olivaceous Warbler Hippolais pallida elaeica and Isabelline Shrike Lanius isabellinus. However, reverse-direction migration need not be the only means by which Siberian vagrants reach northwest Europe in autumn.

Among Eurasian birds, species migrating in an east-west direction are more likely to reverse their migration than are those migrating in a north-south or northeast-southwest direction (Thorup 2004). This difference may be related to the availability of orientation cues on different migratory axes, or to birds using mainly different cues on east-west and north-south axes. Furthermore, the very long distances covered by some displaced birds (up to twice the normal length of journey) throw doubt on the effectiveness of the birds' internal clock in terminating migration (Thorup 2004). It may be the same faulty mechanism that leads them to take a spring direction in autumn.

These considerations also raise the question whether factors other than west-blowing winds could underlie the occurrence of some North American species in Europe. Both spring overshooting and autumn reversed-direction migration have been suggested. Most of the eastern North American coastline runs southwest-northeast. If one draws a line on a globe up the Eastern Seaboard, say from South Carolina to Newfoundland, and then projects that line across the Atlantic, it reaches the British Isles (Figure 10.8). Hence, a bird migrating northeastwards up the Eastern Seaboard in spring and overshooting into the North Atlantic could reach northwest Europe, providing it had sufficient body fuel and favourable winds. Correspondingly, nearly all the North American passerine vagrants that appear in Britain in spring breed well to the northeast in Canada. This phenomenon could thus account for the numerous spring records in Britain of North American sparrows, together with the Brown-headed Cowbird Molothrus ater, Cape May Warbler Dendroica tigrina and Cedar Waxwing Bombicilla cedrorum, all appearing as long-distance spring overshoots.

As for autumn vagrants from North America, some could well be reversed-direction migrants. Such birds would normally migrate from northeastern North America southwestward to Central America, but on reverse migration they could again end up in northwest Europe. Examples of autumn species that might have arrived in this way in Britain include Tennessee Warbler Vermivora peregrina, Hooded Warbler Wilsonia citrina and Sandhill Crane Grus canadensis (Cottridge & Vinicombe 1996). Reversed migration might also account for the autumn appearance at Point Barrow on the northern coast of Alaska of North American species from further south (Table 10.4), for the appearance in British Columbia in September 1889 of a Gray Kingbird Tyrannus dominicensis from the southeastern US and Caribbean region, and for the occasional autumn-winter appearance of Dusky-capped Flycatchers Myiarchus tuberculifer, Greater Pewees Contopus perti-nax, Streak-backed Orioles Icterus pustulatus and Varied Buntings Passerina versicolor in California from regions to the southeast (Robertson 1980).

Reversed migration may also occur among birds that have reached their normal wintering areas, but then simply continue in that direction when they migrate again in spring (equivalent to extending their autumn journey). This may account for the appearance of southern South American birds in North America in autumn. For example, the Fork-tailed Flycatcher Tyrannus savana breeds in southern South America and winters in northern South America, but in spring instead of returning southwestward, some individuals travel northeast through the eastern States, appearing as far north as the Nova Scotian islands (Kaufman 1977). Similarly, on the west coast the Tropical Kingbird T. melancholicus (nominate form), which normally spends the non-breeding season in Central America,

Figure 10.8 Two mechanisms through which eastern North American vagrants might appear in northwest Europe. If a bird flying north in spring up the North American seaboard overshoots into the North Atlantic, it is likely to arrive in northwest Europe if it continues flying along the same great circle route. The same is true for any bird reverse migrating along the same route in autumn. From Cottridge & Vinicombe (1996).

Figure 10.8 Two mechanisms through which eastern North American vagrants might appear in northwest Europe. If a bird flying north in spring up the North American seaboard overshoots into the North Atlantic, it is likely to arrive in northwest Europe if it continues flying along the same great circle route. The same is true for any bird reverse migrating along the same route in autumn. From Cottridge & Vinicombe (1996).

has been found at various localities north to Vancouver, with at least one record in Alaska. At this time of year, the Central American wintering populations should be moving southeast towards their breeding areas, rather than northwest on an apparent reversed migration.

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