The natural geomagnetic field varies over the Earth’s surface between about 0.035 - 0.07 mT and is perceived by certain animals that use it for orientation.

Stefan Maus: They're equipped with very sensitive magnetometers that measure the direction and strength of the magnetic field and that altitude.

"They're just taped on temporarily," Hord said. "we just put the magnets on when they're captured and since they don't know where we take them, they're lost."