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 magneti c north pole. Migratory orientation of snow buntings was recorded usin g two different techniques: orientation cage tests and free-flight rel ease experiments. Experiments were performed under clear skies, as wel l as under natural and simulated complete overcast, Several experiment al 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 prove d to be decisive for the directional selections of the buntings. Fat i ndividuals generally chose southerly mean directions, whereas lean bir ds 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 c lear skies, the buntings failed to respond to either a deflection of t he E-vector direction of polarized light or an experimental depolariza tion of incoming skylight. When tested under natural as well as simula ted overcast, the buntings were still able to select a meaningful mean direction according to their fat status. Similarly, the free-flight r elease test under complete overcast resulted in a well-defined southso utheast direction, possibly influenced by the prevailing light northwe st wind. Clock-shift experiments did not yield a conclusive result, bu t the failure of these birds to take off during the subsequent free-fl ight release test may indicate some unspecified confusion effect of th e treatment.