The orientation system of birds - I. Compass mechanisms

Because of the large distances involved, birds establish contact with their goal indirectly via an external reference. Hence any navigation is a two-s tep process: in the first step, the direction to the goal is determined as a compass course; in the second step, this course is located with a compass . The geomagnetic field and celestial cues provide birds with compass infor mation. The magnetic compass of birds, the sun compass the star compass and the interactions between the compass mechanisms are described in the prese nt paper. Magnetic compass orientation was first demonstrated by testing ni ght-migrating birds in experimentally altered magnetic fields: the birds ch anged their directional tendencies according to the deflected North directi on. The avian magnetic compass proved to be an inclination compass: it does not use polarity; instead it is based on the axial course of the field lin es and their inclination in space, distinguishing "poleward" and "equatorwa rd" rather than North and South. Its functional range is limited to intensi ties around the local field strength, but this biological window is flexibl e and can be adjusted to other intensities. The magnetic compass is an inna te mechanism that is widely used in bird migration and in homing. Its most important role, however, is that of a basic reference system for calibratin g other kinds of orientation cues. Sun compass orientation is demonstrated by clock-shift experiments: Shiftin g the birds' internal clock causes them to misjudge the position of the sun , thus leading to typical deflections which indicate sun compass use. The a nalysis of the avian sun compass revealed that it is based only on sun azim uth and the internal clock; the sun's altitude is not involved. The role of the pattern of polarized light associated with the sun is unclear; only at sunset has it been shown to be an important cue for nocturnal migrants, be ing part of the sun compass. The sun compass is based on experience; sun az imuth, time of day and direction are combined by learning processes during a sensitive period, with the magnetic compass serving as directional refere nce. When established, the sun compass becomes the preferred compass mechan ism for orientation tasks within the home region and homing: in migration, however, its role is minimal, probably because of the changes of the sun's are with geographic latitude. The star compass was demonstrated in night-migrating birds by projecting th e northern stars indifferent directions in a planetarium. The analysis of t he mechanism revealed that the internal clock is not involved; birds derive directions from the spatial relationship of the star configurations. The s tar compass is also established by experience; the directional reference is first provided by celestial rotation, later, during migration, by the magn etic compass. The relative importance of the various compass mechanisms has been tested i n experiments in which celestial and magnetic cues gave conflicting informa tion. The first response of birds to conflicting cues differs considerably between species; after repeated exposures, however, the birds oriented acco rding to magnetic North, indicating a long-term dominance of the magnetic c ompass. Later tests in the absence of magnetic information showed that cele stial cues were not simply ignored, but recalibrated so that they were agai n in agreement with magnetic cues. The magnetic compass and celestial cues complement each other: the magnetic field ensures orientation under overcas t sky; celestial cues facilitate maintaining directions, for which the magn etic compass appears to be ill suited. In view of this, the magnetic field and celestial cues should be regarded as integrated components of a multifa ctorial system for directional orientation.