So called short-stopping has occurred in many species as more food has become available at higher latitudes in the wintering range, either through human activities or climate change. Several populations of Canada Geese Branta canadensis in North America have responded in this way to agricultural changes or to the creation of refuges where food was provided (e.g. Terborgh 1989, Hestbeck et al. 1991), as did
Greylag Geese Anser anser and Common Cranes Grus grus in Europe (Rutschke 1990, Alonso et al. 1991). Other species of waterfowl have shortened their migrations, apparently in response to warmer winters, as open water has become available nearer the breeding areas. This is manifest by increased numbers wintering in northern and eastern parts of Europe, and declining numbers of the same species wintering in the south and west. Yet other waterfowl have shortened their migrations in apparent response to reduced disturbance and predation, as sanctuaries have been established in areas previously open to hunting (Chapter 27).
Shortened migrations in many other species are reflected in the changing distributions of ring recoveries. Among 30 species of short-distance or partial migrants breeding in Germany, a tendency towards wintering at higher latitudes was found in 10 species, and at lower latitudes in three species, although ringing recoveries are affected by changes in human land use and hunting, as well as in climate (Fiedler et al. 2004). More and more European migrants that formerly wintered entirely in tropical and southern Africa are now also wintering in small but increasing numbers in the Mediterranean region. Examples include the Yellow Wagtail Motacilla flava, House Martin Delichon urbica, Osprey Pandion haliaetus, Lesser Kestrel Falco naumanni and White Stork Ciconia ciconia (Table 21.1; Berthold 2001).
In some regions irruptive migrations have become less frequent than formerly, presumably because the birds have become less numerous, or more often remain in their breeding areas year-round or migrate less far. Comparing the nineteenth with the twentieth century, the Pine Grosbeak Pinicola enucleator became a much less frequent visitor to the middle latitudes of Europe. No noticeable invasions of Scandinavian Great Tits Parus major and Blue Tits Parus caeruleus to Britain have occurred since 1977, and no big invasions of Great Spotted Woodpeckers Dendrocopos major since 1974. In Germany, invasions of Blue Tits, Waxwings Bombicilla garrulus and Redpolls Carduelis flammea have also become less frequent (Fiedler 2003). On the other hand, Two-barred Crossbills Loxia leucoptera have appeared in Fennoscandia in increasing numbers and frequency, possibly associated with the increased planting of larch Larix species outside their natural range. Likewise, in eastern North America, Evening Grosbeaks Hesperiphona vespertina have become less numerous, and their invasions less frequent, than previously. This may be associated with reduced outbreaks of Spruce Budworm Choristoneura fumiferana, a favoured summer food, and with increased winter bird feeding by householders.
Some Whooper Swans Cygnus cygnus and Barnacle Geese Branta leucopsis have shortened their migrations in another way, by establishing nesting populations hundreds of kilometres south of their historic breeding range, nearer to wintering areas (Table 21.1). This type of change is more difficult to explain, but may in some cases be due to suitable habitat being created by human activities, to expansion into former range from which the species was eliminated by humans in the past, or simply to unprecedented population increase leading to an expansion of breeding range into lower latitudes.
Such changes often occurred within the lifetimes of individual birds, some of which substantially changed their breeding or wintering areas from one year to the next, so they presumably represented facultative responses to prevailing conditions, rather than genetically driven changes (although genetic change may follow).
Other types of change have also occurred. For example, like many other birds that do not start to breed until they are two or more years old, young White Storks Ciconia ciconia remain in 'winter quarters' through their first summer, or migrate only part way towards breeding areas (Chapter 15). In recent decades, second-summer birds, whose predecessors used to remain in Africa, have returned in increasing numbers to southern Europe to pass the summer. The mean distance of recoveries of second-summer birds from their natal sites in north Germany was 2517 km in 1923-1975 (N=120), reducing to 720 km in 19781996 (Fiedler 2001).
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