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 free-flight 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.