In using celestial cues, long east-west migrations present greater navigational problems than north-south flights because they involve time shifts, as the birds pass through successive time zones. If long-distance migrants using celestial cues to navigate did not allow for time shifts during the course of their east-west journeys, they would make ever greater directional errors, and would thereby veer progressively further off course. The problem created by time shift is greatest at the highest latitudes, where the longitude lines are closest together, requiring more rapid adjustment. Hence, a second presumed function of an internal clock is to measure the changes in timing of sunrise and sunset, as the bird flies long distances west or east. High-latitude east-west or west-east flights are not uncommon, being performed every year, for example, by the many waterfowl and seabirds that migrate along the northern coasts of Eurasia and North America to reach the Atlantic or Pacific Oceans on either side (Chapter 1).
Apart from measuring changes in the time of sunrise and sunset through an internal clock during westward or eastward travel, there is no global cue through which birds are definitely known to detect their longitudinal position. This is in contrast to latitude, which birds could determine at any time, either through the height of the celestial centre of rotation above the horizon or from the angle of magnetic inclination, both of which birds can apparently sense (Emlen 1975, Wiltschko & Wiltschko 1972). But whatever their sensory capacities, birds may be better able to fix their direction when stationary on the ground than when in migratory flight (Lack 1960a), and major changes in direction normally follow stopovers in the journeys. Perhaps birds need time to learn the movement patterns of the sun or stars or local magnetic conditions in a new area in order to be able to fix their position precisely.
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