Animals possess sensory powers that humans lack. Homing pigeons fly with great speed and accuracy when 21 with messages to faraway places. How do pigeons 22 themselves in unfamiliar regions? This remains something of an 23 . The mystery, however, is partly explained by a pigeon's ability to see ultraviolet light, which reveals the sun's position even through clouds. In addition, pigeons can hear sound waves that have traveled hundreds of miles. These waves 24 a pigeon's sense of direction by indicating distant mountains and seas. Pigeons even appear to 25 changes in the earth's magnetic field.