Das Orientierungssystem der Vögel I. Kompaßmechanismen

Abstract

SummaryThe orientation system of birds -I. Compass mechanismsBecause of the large distances involved, birds establish contact with their goal indirectly via an external reference. Hence any navigation is a two-step 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 information. The magnetic compass of birds, the sun compass the star compass and the interactions between the compass mechanisms are described in the present paper. Magnetic compass orientation was first demonstrated by testing night-migrating birds in experimentally altered magnetic fields: the birds changed their directional tendencies according to the deflected North direction. 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 lines and their inclination in space, distinguishing "poleward" and "equatorward" rather than North and South. Its functional range is limited to intensities around the local field strength, but this biological window is flexible and can be adjusted to other intensifies. The magnetic compass is an innate mechanism that is widely used in bird migration and in homing. Its most important role, however, is that of a basic reference system for calibrating other kinds of orientation cues. Sun compass orientation is demonstrated by clock-shift experiments: Shifting the birds' internal clock causes them to misjudge the position of the sun, thus leading to typical deflections which indicate sun compass use. The analysis of the avian sun compass revealed that it is based only on sun azimuth 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, being part of the sun compass. The sun compass is based on experience; sun azimuth, time of day and direction are combined by learning processes during a sensitive period, with the magnetic compass serving as directional reference. When established, the sun compass becomes the preferred compass mechanism 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 arc with geographic latitude. The star compass was demonstrated in night-migrating birds by projecting the northern stars in different directions in a planetarium. The analysis of the mechanism revealed that the internal clock is not involved; birds derive directions from the spatial relationship of the star configurations. The star compass is also established by experience; the directional reference is first provided by celestial rotation, later, during migration, by the magnetic compass. The relative importance of the various compass mechanisms has been tested in experiments in which celestial and magnetic cues gave conflicting informati...

The evolution of bird migration—a synthesis

“…When this happens in nature, these individuals are able to migrate towards the equator in the non-breeding season, as suggested for the southern populations of booted eagle and European beeeater (Harrison et al 1997) and shown for southern-bred white storks by ring recoveries and satellite telemetry (McLachlan 1963;Underhill 2001). Birds use, among other compasses, a magnetic inclination compass for orientation (Wiltschko and Wiltschko 1999b). According to this compass, Palearctic migrants are not moving southwards according to their endogenous migration program but equatorwards to their non-breeding grounds.…”

Is There Visual Lateralisation of the Sun Compass in Homing Pigeons?

“…In semi-natural and natural settings, natural spatial cues such as the sun compass constitute an important and often dominant source of information for birds challenged to localise a food reward in an outdoor arena, or to orient towards a goal [15][16][17][18][19][20][21][22][23][24]. Information useful for determining an absolute direction using the sun reference are processed through the visual system.…”

“…In fact, sun compass orientation requires the observation of the sun azimuth in order to compute an angle to take with it on the basis on the information about the time of the day provided by the internal clock. A clear demonstration of the use of the sun compass by birds is the consistent deviation from the goal direction shown by birds subjected to phase-shift; an altered light-dark cycle is used to manipulate the circadian rhythm of test birds, thereby predictably altering time-compensated interpretations of the sun compass direction [20][21][22].…”

“…Despite the importance of the sun compass mechanism in birds' spatial behaviours [15,17,18,[20][21][22], lateralisation phenomena in relation to sun compass-mediated spatial tasks have been poorly investigated. Few exceptions are represented by studies conducted with homing pigeons subjected to unilateral hippocampal lesions.…”

Avian compass systems: do all migratory species possess all three?

“…A reader of almost any more or less recent review on avian orientation (Wiltschko and Wiltschko , Muheim et al , Fraser ) is most likely to learn that migrating birds are known to possess at least three mechanisms of direction finding, namely the solar compass (Kramer , Wiltschko , ), the stellar compass (Emlen , , ) and the magnetic compass (Wiltschko and Wiltschko ). These three compass systems have been shown to exist in migrating birds, their existence has been independently replicated multiple times and can now be considered an established fact rather than a hypothesis.…”