magnetic field using large magnetic coils, such as modified Helmholz coils
(Wiltschko and Wiltschko 1995), the perception of the geomagnetic field can be
manipulated. In a similar way sun compass orientation has been investigated by
shifting the position of the sun using mirrors, and by using filters to shift the
alignment of polarized light and to depolarize the incoming light. Opaque
Plexiglas sheets placed on top of the cages are used to screen off visual cues. By
shifting the birds’ internal time sense relative to the natural dark–light cycle
using an artificial dark–light cycle, the function of the birds time-compensated
sun compass can be investigated.
In studies of the functional characteristics of the birds’ magnetic sense various
techniques have been applied, such as exposure to strong (0.5–1 T) magnetic
pulses and thereafter observation of the birds’ orientation in cages, as well as
neurophysiological recordings during magnetic field manipulations (for review
see Wiltschko and Wiltschko 1995). In the search for a magnetic sensor con-
taining magnetite, histology techniques, magnetic force microscopy, and a
Superconducting Quantum Interference Device (SQUID) are used.
In conditioning experiments a bird is trained to detect a feeder associated with
a particular stimulus (magnetic, visual), and then the bird’s ability to use this cue
is challenged with only the stimuli present at randomized locations.
7.6.3Displacement experiments
Birds and other animals have been suggested to navigate by using either a combi-
nation of two geomagnetic parameters, field intensity and the angle of inclination
Orientation and navigation| 173Fig. 7.6Adult white-crowned sparrow in orientation cage (Emlen
type) at the magnetic North Pole. The sloping walls are covered
by type-writer correction paper. Photo: Susanne Åkesson.