Correction for drift

Drifted birds that have settled usually leave their drift areas when the wind slackens or veers to a more favourable direction. Four types of evidence indicate that birds can detect their displacement and attempt to get back on course. First, radar observations have shown re-oriented movements after drift has occurred (Myres 1964, Evans 1966b, Able 1977). Second, some radio-tagged birds tracked as they were displaced by wind subsequently re-directed themselves to their normal route (for Bald Eagle Haliaetus leucocephala see Harmata 2002; for Whooper Swan Cygnus cygnus see Pennycuick et al. 1996), and some birds ringed as drifted migrants were later recovered in localities closer too, or within, their regular range, including migration areas (Evans 1968; for extreme examples see Table 10.3). Third, known drifted birds trapped and tested in orientation cages did not head mainly in their normal migration direction, but in whichever direction would have got them back on their normal route (Evans 1968, Able 1977). Such birds tested on the ground showed the same appropriate directional preferences as birds tracked by radar (Evans 1968, Able 1977). Fourth, birds caught on migration and transported 450 km westward off route compensated for their displacement when tested in orientation cages (Rab0l 1969a, 1994). This was true of juveniles, as well as of adults. Hence, although other evidence suggests that juveni les inherit only a general compass direction to guide them on their first journey (Chapter 9), they nevertheless seem able to recognise and correct for any drift to which they are subjected. Unless they get far from land in continuing adverse winds, therefore, migrants are unlikely to be drifted long distances off course before they re-orientate.

When correcting for effects of wind drift, birds take different directions in different regions, according to local geography. Off eastern North America, autumn migrants drifted over the sea generally turned at dawn and headed northwest, which in general gave the shortest route to the predominantly southwest-northeast running coastline (Baird & Nisbet 1960, Able 1977). Migrants drifted from continental Europe to the east coast of England re-orientated southeast, which would have got them back on their normal southwest route through western Europe (Evans 1968), while those blown beyond the west coast of Britain over the Atlantic usually back-tracked until they reached land (Myres 1964). Similarly, vagrants to the Farallon Islands off California often approach the islands from the west in the mornings, suggesting that they were back-tracking after having found themselves over the sea at dawn (Robertson 1980). Much of this re-orientation by night migrants occurs in daytime, and the birds may often be able to see the coast from the altitude at which they normally fly.

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