To summarise so far, experiments indicate that birds can use a number of different compasses for orientation during long-distance migrations, based on information from the sun and related pattern of skylight polarisation, from star patterns and from the earth's magnetic field. These different cues would normally give the same directional message. The use of the sun compass requires a time-compensation mechanism, through which the bird can allow for changes in the sun's position during the day, while the rotation centre of the night sky indicated by the stars gives the direction towards the geographical poles (Emlen 1975). Geomagnetic compass courses are given by the angle of inclination (varying from horizontal at the equator to vertical at the poles). The reliability of these compasses might vary between regions, seasons and local conditions, so that for instance the sun compass cannot be used if the sky is totally overcast, the star compass might not be visible during the round-the-clock daylight in high-latitude summers, and a magnetic compass based on the angle of inclination is unusable around the geomagnetic poles and the geomagnetic equator (Akesson et al. 2001). All the known compasses used by migratory birds seem to be modifiable by experience during early development, or later. Bi-coordinate navigation could be provided by any two non-parallel gradients, and in theory the different gradients could be provided by different types of cues, for example one coordinate being based on a celestial cue and another on a magnetic cue. Latitude can be fixed by both celestial and magnetic cues, but longitude seems much more difficult to determine. Displacement experiments with White-crowned Sparrows Zonotrichia leucophrys in arctic North America indicated that a combination of geomagnetic and celestial information might be used to define longitude, but the precise mechanism remains unclear (Akesson et al. 2005).
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