Migratory flights in directions opposite to those expected occur commonly in both spring and autumn, and have already been mentioned above. They have been explained as responses to adverse weather (with birds turning back when conditions ahead are bad), as orientation errors, or as attempts to correct previous orientation errors or wind drift or overshooting (with birds back-tracking to a point en route, for example see Pennycuick et al. 1996). When reverse movements occur at coasts, they have also been interpreted as attempts by birds with small fuel reserves to feed inland and increase their reserves before setting out over water (Alerstam 1978b). By moving inland, the argument goes, the birds avoid the high competition, depleted food supplies or predation risk caused by the build-up of birds near the coast. Inland, birds can accumulate body reserves more rapidly and safely. To judge from recoveries of 20 passerine species ringed in southern Sweden in autumn, reverse movements varied between 9 and 65 km, and species with small fat reserves were more likely to perform reverse movements than were species with larger reserves (Akesson et al. 1996). Moreover, among Chaffinches Fringilla coelebs and Bramblings F. montifringilla in the same area, the peak in reverse movements occurred about 3.5 hours after the early morning departure in the normal direction; and the average weights of reverse migrants and of birds lingering at the coast were lower than those of birds of the same species that proceeded in the normal direction (Lindstrom & Alerstam 1986).
In spring, if birds encounter snow or other bad weather en route or after arrival in breeding areas, they often retreat for some distance in the direction of their wintering areas, providing that body reserves permit (e.g. Gatke 1895, Williams 1950, Svardson 1953, Gauthreaux & LeGrand 1975). Many thousands of birds can be involved in such movements. The birds can advance again with the next warm front, and at times of alternating mild and cold periods, back-and-forth shuttle movements sometimes ensue until birds can eventually settle in their nesting areas. The same occurs among birds that breed on high mountains, which having settled on their breeding areas in spring, move down-slope during spells of bad weather, and back again when conditions improve. In northern Europe, the Lapwing Vanellus vanellus is one of the earliest species to return each spring, and is well known for reverse migration. The first individuals to arrive on the coast of Finland in spring often turn back on the same day if they encounter cold and snow. One spring, more than 10 000 individuals per day were counted moving southwest across one 10-km stretch of coast (Vepsalainen 1968). In autumn, when birds generally migrate towards warmer climes and better feeding conditions, adverse weather is likely to play a smaller role, with journeys less disrupted. Although reverse movements often occur on the same days as normal movements, they are often at different altitude, with birds selecting wind conditions appropriate to their flight direction.
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