The most obvious way in which birds and other animals could find their way around on a day-to-day basis is by use of landmarks or other consistent features of their home areas. This explains how some migrant birds manage to return to exactly the same nesting places year after year. But such features are useful only in familiar areas, and when moving over longer distances into unknown terrain, a reliable geographical reference system is needed for navigation. At least two types of factors can act as compasses - celestial and geomagnetic - and both are used by birds as directional aids (for reviews see Emlen 1975, Able 1980, Wiltschko & Wiltschko 1995, 2003, Berthold 1996, Akesson 2003). In migratory birds, compasses based on the sun (and various sunset cues), stars and magnetic information have been studied in detail, but a prior requirement for using any compass is that the bird should 'know' beforehand - either by inheritance or experience - what direction it has to take. Also, effective use of any of these compasses requires a period of learning, and frequent revision as the bird continually changes location while on migration.
One feature of celestial cues, such as the sun and stars, is that they appear to change in position through each 24-hour cycle, as the earth spins on its axis. In the northern hemisphere, the sun lies in the south and moves during the day from east to west, and at night the stars rotate anticlockwise around the geographical north. In the southern hemisphere the sun lies in the north and moves from east to west, while the stars rotate clockwise around the geographical south. In using the sun and related factors in direction finding, therefore, birds in both hemispheres must allow for time of day. But the same is not necessarily true for star patterns if they are used solely to indicate geographical north or south, determined by the centre of rotation of the night sky. Time-keeping depends on the 'internal clock', kept to time by the regular day-night cycle.
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