Bird Migration Across a Strong Magnetic Anomaly

ABSTRACT Migratory flights of birds across a strong magnetic anomaly were recorded by tracking radar and, during the daytime, by simultaneous visual observations. The anomaly in central Sweden is about 12 km long and a few kilometres wide, with the total magnetic intensity at low altitude exceeding the normal field by up to 60 %, according to aeromagnetic measurements. Migrants change altitude, most often by starting to descend, to a significantly greater extent over those parts of the anomaly where the magnetic field intensity is abnormally high, and particularly where the magnetic inclination is steepest, than over other parts of the study area and at another study area with a normal geomagnetic field. The descents lasted on average about 2min, with a mean angle of descent of 2·6°, leading to a height loss of about 100 m before level flight was resumed. Flock formations were repeatedly broken up during these temporary descents. The changes in flight altitude were associated with gradients in magnetic inclination along the birds’ flight paths across the anomaly. This supports the possibility that birds use the geomagnetic field and associated gradients for continuous guiding and recording of migration. Additional visual observations of migrating birds at the lowest altitudes over the magnetic anomaly suggest that birds sometimes become briefly disturbed (clear weather) or disoriented for longer periods, because of difficulties in finding their way out of the anomaly area (poor visibility).