Many animals use environmental cues to migrate long distances. The navigational achievements of homing pigeons are legendary. They can home—that is, find their way back to the place where they are housed and fed—over extremely long distances, even of more than a thousand miles. It seems that they use a wealth of cues when homing. A compass may tell you which way is north, but you must also have map information—you must know where you are with respect to your goal—if the compass is to be useful for finding a particular location. So pigeons must have both compass and map senses. That map sense is presumably based on landmarks. Given the long distances the pigeons travel, these landmarks must vary in their type and probably include olfactory as well as visual cues.

The pigeon compass relies on different kinds of information. For example, pigeons can navigate during the day using the sun as a compass, and at night using the stars. And, like Aquaspirillum, they can detect Earth’s magnetic field. The magnetic navigation system may be important to pigeons on cloudy days. Researchers have performed experiments with pigeons wearing magnetic helmets, which disrupt the pigeons’ ability to detect Earth’s magnetic field. On cloudy days, the pigeons wearing the helmets are unable to navigate home. On sunny days, however, the pigeons’ sun compass overrides the erroneous magnetic information of the helmets.

The sun compass requires information about time as well. Every hour, the sun moves 15 degrees through the sky. To determine where north is, a pigeon needs information from the sun as well as some way to keep time. It turns out that pigeons have a clock—a biological one.