Orientation of Snow Buntings (Plectrophenax nivalis) close to the Magnetic North Pole

Orientation experiments were performed with first-year
snow buntings (Plectrophenax nivalis) during their autumn
migration in a natural near-vertical geomagnetic field
approximately 400 km away from the magnetic north pole.
Migratory orientation of snow buntings was recorded using
two different techniques: orientation cage tests and freeflight
release experiments. Experiments were performed
under clear skies, as well as under natural and simulated
complete overcast. Several experimental manipulations
were performed including an artificial shift of the E-vector
direction of polarized light, depolarization of incoming
light and a 4 h slow clock-shift experiment. The amount of
stored fat proved to be decisive for the directional
selections of the buntings. Fat individuals generally chose
southerly mean directions, whereas lean birds selected
northerly headings. These directional selections seemed to
be independent of experimental manipulations of the
buntings’ access to visual cues even in the local near-vertical
magnetic field. Under clear skies, the buntings
failed to respond to either a deflection of the E-vector
direction of polarized light or an experimental
depolarization of incoming skylight. When tested under
natural as well as simulated overcast, the buntings were
still able to select a meaningful mean direction according
to their fat status. Similarly, the free-flight release test
under complete overcast resulted in a well-defined
southsoutheast direction, possibly influenced by the
prevailing light northwest wind. Clock-shift experiments
did not yield a conclusive result, but the failure of these
birds to take off during the subsequent free-flight release
test may indicate some unspecified confusion effect of the
treatment.