Examples of changes from sedentary to migratory behaviour are somewhat less common, and are generally associated with an extension of breeding range into higher latitudes. For example, the European Serin Serinus serinus was once restricted to the south of Europe where it is resident, but in the early twentieth century it spread north, where it became migratory. In more recent years, with milder winters, this migratory population has become partially resident (Berthold 1999). Likewise, since the nineteenth century, many bird species have spread north in Fennoscandia, including the Northern Lapwing Vanellus vanellus, Starling Sturnus vulgaris, Eurasian Blackbird Turdus merula and Dunnock Prunella modularis. In their newly colonised breeding areas they are essentially migratory, whereas further south they are partial migrants or residents (Schuz et al. 1971).
Common Starlings Sturnus vulgaris introduced to North America at the end of the nineteenth century supposedly came from resident British stock. They were initially sedentary in the eastern USA but, in connection with range expansion, different proportions of migrants appeared in different regions (Kessel 1953, Dolbeer 1982). Similarly, as Cattle Egrets Bubulcus ibis spread north through North America, they remained resident in Florida and other southern parts, but became migratory further north (Root 1989, Maddock & Geering 1994). During the nineteenth century, Snowy Egrets Egretta thula were eliminated by human persecution from the northern parts of their breeding range in North America where they were migratory, and survived only in the southern parts where they were resident. As the remnant resident population has recovered in recent decades, the birds have spread northwards, where they have become migratory in the newly colonised areas (Rappole 2005). Severe range contractions are likely to have eliminated the migratory or resident sectors from other species in which both types of behaviour were once represented, one or other type of behaviour being regained if the remnant population expands to re-occupy its former range.
It is not only the migrations themselves that can change, but also the sex and age differences within populations. The House Finch Carpodacus mexicanus was introduced from California, where it is resident, to eastern North America. Within less than 30 years in its new home, as numbers grew, greater proportions of individuals migrated; they also migrated progressively further, and females further than males (Belthoff & Gauthreaux 1991, Able & Belthoff 1998). Moreover, the birds in western parts of this eastern distribution now show more north-south migration directions than the more eastern birds which migrate northeast-southwest (Brewer et al. 2000). These changes are almost certainly based partly on genetic changes, evolved under the action of natural selection.
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