Migration along orthodromic sun compass routes by arctic birds

Flight directions of birds migrating at high geographic and magnetic latitu des can be used to test bird orientation by celestial or geomagnetic compas s systems under polar conditions. Migration patterns of arctic shorebirds, revealed by tracking radar studies during an icebreaker expedition along th e Northwest Passage in 1999, support predicted sun compass trajectories but cannot be reconciled with orientation along either geographic or magnetic loxodromes (rhumb Lines). Sun compass routes are similar to orthodromes (gr eat circle routes) at high latitudes, showing changing geographic courses a s the birds traverse longitudes and their internal clock gets out of phase with Local time. These routes bring the shorebirds from high arctic Canada to the east coast of North America, from which they make transoceanic fligh ts to South America. The observations are also consistent with a migration Link between Siberia and the Beaufort Sea region by way of sun compass rout es across the Arctic Ocean.