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.