Migration routes in the Arctic and Antarctic are of special interest. For it is at high latitudes that great circle routes bring the greatest proportional reductions in distance. It is also near the poles where the longitude lines are closer together that migrants become exposed to the most rapid time-shifts, and where birds are faced with difficulties in using any sort of recognised compass. Use of a sun compass brings problems of time compensation during rapid longitudinal (east-west) displacement, but is still usable. Use of a star compass is not possible in the polar summer because stars are not visible for months on end. And a magnetic compass is unreliable in a wide region around the north and south magnetic poles owing to the rapid changes in declination of the geomagnetic field in those regions, as well as the very steep angles of inclination. After considering the possibilities, Alerstam & Gudmundsson (1999) concluded that shorebirds tracked by radar off northern Siberia were using sun compass orientation as they travelled along apparent orthodrome-like routes. Orientation experiments at high latitudes revealed that birds made use of celestial cues (Ottosson et al. 1990, Akesson et al. 2001), but could also detect magnetic information, despite the high angle of inclination (Muheim et al. 2003). White-crowned Sparrows Zonotrichia leucophrys under simulated overcast could select a magnetic compass course where the inclination angle was less than 3° from the vertical, implying a very accurate receptor. Interestingly, this species does not normally live at such high latitudes.
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