Reverseddirection Migration

Another type of navigational error that could result in a one-step change in migration direction is reversed-direction migration, in which some young birds leave the breeding range in the direction opposite to usual; that is, they take their spring direction in autumn, or vice versa (Nisbet 1962, Rab0l 1969b, 1976). This could be regarded either as an error in timing or in navigation (correct bearing, but wrong side of both north-south and east-west axes). Among European birds, the Barred Warbler Sylvia nisoria and Red-breasted Flycatcher Muscicapa parva appear each autumn across northern Europe from Iceland to Finland. They breed far to the southeast and east-southeast and migrate more or less southeast and east-southeast to East Africa and India respectively. Their autumn occurrence in northern Europe thus involves a displacement almost directly opposite to their normal direction for autumn migration. Similarly in North America, the Yellow-breasted Chat Icteria virens breeds over much of the USA north to about 42°N. In autumn, it migrates to the southwest, but is seen in surprising numbers along the Eastern Seaboard from New York City to Nova Scotia and Newfoundland, up to 1500 km northeast of its breeding range. Again, this has been attributed to a reversal of the normal migration direction (Nisbet 1962).

One of the most striking supposed examples of reverse-direction migration is provided by the Pallas's (Lemon-rumped) Warbler Phylloscopus proregulus which breeds in eastern Asia but is now regularly recorded in late autumn in northwest Europe. Again, these birds could be making a simple mistake of 180° in their migratory flight; the great circle route projected from the normal wintering area through the breeding area would reach northwest Europe (Rab0l 1969; Figure 10.6). In fact, almost any species which normally migrates in autumn from western or central Siberia to Southeast Asia could end up in northwest Europe if its migration was reversed (Cottridge & Vinicombe 1996). The area where such species would be expected on reverse migration has been termed the 'vagrancy shadow'. The presence of these species in northwest Europe cannot be attributed entirely to wind drift, because such long journeys are likely to be spread over several weeks, during which the birds could correct for their displacement if they perceived themselves as being off course.

The proposed reality of reverse or mirror-image misorientation stems from observations that, in certain situations, specific large-angle misorientations seem

Figure 10.6 Hypothetical reversed 'great circle' migration route of Pallas's Warbler Phylloscopus proregulus. From Rab0l (1969b).

more common than small or intermediate deviations from the normal migration course. Moreover, misorientations could best explain why species that breed in far away areas turn up more frequently than other closely related species that breed commonly only a short distance away. For example, Pallas's Warblers Phylloscopus proregulus from Siberia are much more frequent in Britain than are Bonelli's Warblers Phylloscopus bonelli which breed as close as France.

If reversed-direction migration were a real phenomenon, rather than an untested hypothesis, it would limit the variety of species expected in any given area to those whose reversed routes passed through that area. The vagrancy shadows of other species would fall elsewhere. Take as examples the Red-breasted Flycatcher Ficedula parva and Collared Flycatcher F. collaris, which have similar breeding distributions in Europe (although the former also extends further east). In autumn, most Red-breasted Flycatchers head southeast for southern Asia, whereas Collared Flycatchers head south for wintering areas in East Africa. The much more frequent autumn occurrence of Red-breasted Flycatchers in northwest Europe (more than 2000 records in Britain up to 1995) compared with Collared Flycatchers (19 records up to 1995) has been attributed to reversed-direction migration of the former (Figure 10.7, Cottridge & Vinicombe 1996). This difference in occurrence between species is greater than expected from the respective sizes of their geographical ranges (and hence population sizes). And because almost all recorded individuals were juveniles in their first autumn, we can exclude persistence after spring overshooting as a cause of their presence.

Many apparent examples of long-distance reversed-direction migration are based on the assumption that migration routes normally follow a great circle. Such routes represent the shortest distance between two points on the earth's surface, but it is by no means certain that birds follow such routes, and to my knowledge they have not yet been convincingly demonstrated for any longdistance migrant over most or all of a migration route (Chapter 9).

Red-breasted Flycatcher

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Week of the year Collared Flycatcher

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