Age and wind drift

In general, juvenile birds are more often drifted off course by crosswinds than adults are, and many of the 'big days' at coastal observatories occur when the wind has a strong easterly or westerly component. In many bird species, apparently as a result of greater drift, disproportionately more juveniles turn up at coastal sites than at inland ones, as often noted among passerines in both Europe and in North America (Murray 1966, Ralph 1971, Smith & Schneider 1978). In addition, about 95% of all the raptors seen in autumn at Cape May on the New Jersey coast are birds of the year, compared with 50% of those seen at Hawk Mountain in inland Pennsylvania (Allen et al. 1996).

The greater effect of crosswinds on juveniles than adults is also evident from radio-marked birds tracked on autumn migration from satellites. Among Ospreys Pandion haliaetus and European Honey Buzzards Pernis apivorus migrating between Europe and Africa, juveniles were drifted off course to a much greater extent than adults, and followed a much more zig-zag day-to-day route to winter quarters (Hake et al. 2003, Thorup et al. 2003). This suggested that adults were better able to detect and compensate for wind drift than juveniles. Perhaps the previous experience of the adults and their familiarity with landmarks below helped them to maintain a straighter course. Adult birds may also be better at selecting for migration days with favourable winds, avoiding days with crosswinds. In addition, however, crosswind compensation may be of less value to juveniles than adults, because, being on their first migration, juveniles have no specific prior winter site as their destination. Third, some of the juveniles may be such poor navigators that they get lost on their first migration, and do not survive to be represented in the adult population. These age-dependent effects of wind may help to explain some of the contradictory findings on wind drift reported from radar studies (Chapter 4).

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