Experimental studies of the development of migratory orientation mechanisms

Summary Recent experimental studies (since ca 1985) on the ontogeny of orientation mechanisms in migratory birds are reviewed. The processes and interactions are synthesized into a framework that may help identify critical research questions. Birds that grow up in the earth's magnetic field develop the ability to perform appropriate migratory orientation even in the absence of any experience with relevant visual cues. In two species, large changes in direction during the course of migration seem to be controlled by an endogenous time program. In one of these, the pied flycatcher (Ficedula hypoleuca), the correct magnetic orientation seems to occur only when the magnetic fields appropriate to the latitudes encountered en route were experienced at the proper seasonal time. The magnetic compass may be modified by visual experience with either the daytime or night sky. Celestial rotation may be the calibrating reference in this case, as it is in the development of the star compass. Young Savannah sparrows (Passerculus sandwichensis) learn to perform compass orientation at sunset based on polarized skylight. This compass capability seems to be calibrated by magnetic directions. Some problems of experimental design and the interpretation of results from experiments on development are discussed.     

Magnetic inclination compass: A basis for the migratory orientation of birds in the Northern and Southern Hemisphere

We conducted orientation experiments with Silvereyes,Zosterops lateralis, Australian passerine migrants, to see whether birds living in the Southern Hemisphere in a magnetic field with an upward inclination orient in the same way as birds in the Northern Hemisphere that experience a downward inclination of the magnetic field. Tested indoors in the local geomagnetic field, the birds preferred southerly directions corresponding to their migratory direction in spring. In a magnetic field with a reversed vertical component, they reversed their directional tendencies. This shows that the magnetic compass of Silvereyes also functions as an inclination compass based on the inclination of the field lines instead of the polarity.     

Magnetic compass orientation in the subterranean rodentCryptomys hottentotus (Bathyergidae)

Summary To test whether mole-ratsCryptomys hottentotus were able to use the magnetic field for orientation, laboratory experiments were conducted which were based on the animals' spontaneous tendency to build their nests at the same position in a circular arena. In the local geomagnetic field, the animals preferred the SE-sector. When magnetic north was turned by 120^o or by 180^o, the mole-rats changed their nest position accordingly. This clearly shows that they can use the magnetic field for direction finding.     

A magnetic pulse leads to a temporary deflection in the orientation of migratory birds

Migratory Australian Silvereyes were treated with a strong magnetic pulse designed to alter the magnetization of the small magnetite particles that are found in birds' heads. Prior to the treatment, the birds preferred the northeasterly migratory direction. The pulse initially resulted in a 90° clockwise shift of orientation; however, within about a week, the birds seemed to return to their original headings. These findings, which seem to suggest an involvement of magnetite in migratory orientation, are in contrast with previous findings which indicated that it is a light-dependent process. They are discussed in view of the current concepts on magnetoreception and on the role of magnetic information in avian orientation.     

Evolutionary aspects of orientation and migration in birds

Summary Evolutionary aspects of avian migratory orientation and navigation are reviewed. A theoretical approach to the evolution of complex (endogenously programmed) migratory behavior is presented using the comparative method of ordering a progression in existing behavioral characteristics from dispersal, to facultative migration, to obligate migration.