Some birds on their migrations also cross substantial stretches of sea-ice. For example, the many shorebirds that fly between northern Siberia and northern North America en route to wintering areas in South America have to cross 1800-3000 km of the Arctic Ocean which is almost totally ice-covered (Alerstam & Gudmundsson 1999). The main participants include Pectoral Sandpipers Calidris melanotos and Grey Phalaropes Phalaropus fulicaria. Nevertheless, such flights would seem to offer less of a challenge than overwater flights of similar length, because the birds can at least stop and rest in emergencies. Many arctic and Antarctic seabirds cross more than 100 km of sea-ice in order to reach their breeding areas in spring, including some penguin species which have to walk. A bigger challenge is presented by the Greenland ice cap, which is up to 1000 km across, and reaches more than 2.5 km above sea level at its highest points. It is crossed regularly by shorebirds and waterfowl on their migrations between breeding areas in northeast Canada or west Greenland and their wintering areas in Europe or Africa. Participants include Brent Geese Branta bernicla and White-fronted Geese Anser albifrons, which have been shown by radio-tracking to stop periodically on the way up (Gudmundsson et al. 1995, Fox et al. 2003), and apparently also Ruddy Turnstone Arenaria interpres, Red Knot Calidris canutus, Ringed Plover Charadrius hiaticula, Purple Sandpiper Calidris maritima and Northern Wheatear Oenanthe oenanthe. Most other birds that breed in western Greenland migrate to North America via the Davis Strait rather than to Europe.
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