Limiting Conditions for Effects of ELF on Free Flying Migrant Birds

Bird migration through an imposed ELF field was studied at the U.S. Navy Wisconsin Test Facility at Clam Lake, Wisconsin. More than 15,000 birds were followed with a small, mobile, short range, high resolution search radar, the ORNITHAR, as they flew over the WTF antenna system which was activated in pre-determined sequences without the knowledge of the observers in the mobile radar. Observations in the vicinity of the transmitter at the WTF revealed a significant association of several migratory behavior variables with antenna state. The most regular of these was a 5 deg. to 20 deg. deviation in the track direction of migrants when the north-south antenna leg was activated. This and other effects did not occur on all nights of observation and neither the occurrence not the magnitude of the effect could be predicted from weather variables or direction or intensity of migration. Subsequent radar observations near the WTF antenna but remote from the transmitter site failed to reveal any effects regularly associated with antenna state such as those noted near the transmitter. The factors responsible for the observed effects near the transmitter site are not known but may still be due to an electromagnetic field.