Migratory. Small numbers winter within maritime climatic zone of western Europe north to Britain (only irregularly, in mild seasons, in Low Countries and central Europe) and around Mediterranean basin. However, majority winter in Africa south to Cape Province, or (eastern populations) across southern Asia to Melanesia and Australia. Basically a freshwater species at all seasons, migrating overland on broad front, where necessary crossing mountains or deserts. Aggregations at favoured staging areas such as reservoirs or sewage-farms exceptionally reach 200 in autumn , but birds normally migrate singly or in very small groups.
For European populations, migration basically SSW-SW in autumn, reversed in spring. Birds ringed Finland and Sweden cross Europe from Baltic, Czechoslovakia, and Italy west to North Sea countries, France, and Iberia. 9 Fenno-Scandian recoveries from east German passage ringing were in Finland (7) and Sweden (2); 19 Fenno-Scandian recoveries from west German ringing were in Finland (3), Sweden (13), and Norway (3); 15 British-ringed migrants subsequently found in Fenno-Scandia were in Sweden (4) and Norway (11). These larger passages of Finnish birds through east Germany, Swedish through West Germany, and Norwegian through Britain are consistent with standard direction of European passage. Thus migrants ringed north-west Europe and recovered further south had moved through France and Iberia into Morocco; German migrants found in Italy had mainly been ringed in Bayern and Sachsen. Birds ringed Czechoslovakia recovered mainly in Italy (on autumn passage), with 3 in southern France, plus one August migrant found Leningrad in July. European winter (December-February) recoveries in Netherlands (Danish-ringed bird), western France (from west Germany and Britain), Spain (from Denmark, west Germany, and Britain), Portugal (from Sweden), and Italy (from Czechoslovakia and Austria).
14 birds ringed in Europe (east to Finland, Poland, and Czechoslovakia) found south of Sahara, September April (and one in July); these in Mali, Guinea-Bissau, Guinea, Sierra Leone (3), Ivory Coast (2), Ghana (5), and Nigeria,i.e. concentrated in "bulge" of West Africa, as to be expected from south-west autumn movement through Europe. This suggests that birds wintering in central, eastern, and southern Africa come from more easterly populations, and the few recoveries from Afrotropical ringing are consistent with this: central Zaïre (April) to Vladimir (April); Kenya (April) to Perm, 54°08'E (May), 6500 km in 44 days; Zambia (August) to Kalinin (September) and Gorkiy (April); Zimbabwe (February) to Vologda (May) and Perm, 54°18'E (May); Also, one ringed Natal (December) found Sudan the following April.
Autumn passage can begin late June (perhaps failed breeders), but more usually early July; possibly a little later in north-east Europe, where breeding season retarded in comparison with temperate latitudes. In west Germany, adult passage throughout July-August, with peak in late July, and juvenile passage (more protracted) late July to late September, peak in August. Age differences initially less pronounced in Britain, where adults and juveniles trapped in equal proportions from beginning of July to mid-August, though thereafter juveniles predominate, markedly so in September. 2 Finnish juveniles reached southwest Germany by 1 August, Italy by 3 August. Only small proportion of adults begin wing moult in western Europe, and Russian adults arrive unmoulted in East Africa. Hence off-passage stops at staging areas --average 18.7 days in Belgian study--are for feeding and fat deposition. In British study, most birds departed after 50% weight increase, but minority (mostly adults) doubled weight and therefore capable of long continuous flight. Trans-Saharan passage through central Chad at peak late August to early September; major arrivals in West and East Africa August-September, relatively early due to few birds moulting further north. Passage virtually completed in October.
Return movement begins March, when South African birds double their mean winter weight. Trans-Saharan passage occurs from late March or early April, e.g. Smith (1968b) for south-east Morocco, Cramp and Conder (Conder 1970) for southern Libya. Vanguard reaches Europe in first half April, and main passage there mid-April to mid-May. In temperate Europe, breeding grounds reoccupied in second half April (15-20 April in England), but not until second half May or even early June in Lappland and northern Russia. Many immatures summer in Africa, and may be responsible for exceptional nesting records there.
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