Changes In Migratory Directions

In addition to the Blackcap Sylvia atricapilla mentioned in the previous chapter, changes in the direction of migration, leading to the adoption of new wintering areas, were recorded in several species in the last century (Table 21.1). For example, Little Egrets Egretta garzetta breeding in southern France migrated southward, some crossing the Sahara to winter in the Afrotropics. But from the 1970s, increasing numbers began to migrate northwest to winter in northern France, southern Britain and Ireland (Marion et al. 2000). Some later became resident in these areas, and from the 1990s started to breed there. Similarly, Lesser Black-backed Gulls Larus fuscus from Europe have begun increasingly to winter on the coasts of eastern North America, with records from Nova Scotia to Florida, a change which requires a much stronger westerly component in the directional preferences. Almost certainly, such marked directional changes have involved genetic changes, as confirmed for the Blackcap by breeding and direction-testing in captivity (Chapter 20).

A different type of change is shown by those northern hemisphere species introduced to the southern hemisphere, which have reversed the direction of their spring and autumn journeys, as appropriate, so that they continue to winter in lower rather than in higher latitudes. This is true, for example, of the European Goldfinch Carduelis carduelis and others introduced from Europe to New Zealand in the nineteenth century, and also for the White Stork Ciconia ciconia, which is thought to have colonised South Africa naturally in the 1930s, and now migrates north to winter in equatorial Africa (Underhill 2001).

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