Nocturnal migrants often set off around dusk, when celestial cues related to the sun (such as sunset position, horizon glow, and skylight polarisation pattern) are clearly visible, and when at the same time the star pattern is gradually emerging. They could therefore make use of all these celestial cues within a relatively short period. The fact that migratory birds keep flying in the same direction over the transition from day to night or night to day (e.g. Myres 1964) implies that they can switch between sun and stars for navigation, or that they rely on some other cue, such as the earth's magnetic field, to maintain their course. In addition, some arctic species which prefer to migrate by night at lower latitudes necessarily migrate in daylight at high latitudes in summer. Moonlight can hinder the use of star patterns and produce the same disturbing effects as cloud (Sauer 1957). The moon itself seems to play no obvious role in bird orientation.
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