Distant Landmarks Used for Navigation by Homing Pigeons.

How animals navigate using single or several sensory modalities remains disputed. Humans often use distant landmarks such as mountain peaks as visual cues for orientation. Whether animals can also rely on distant visual cues while excluding other modalities, such as smell or visual maps, is difficult to test under natural conditions. We investigated this by releasing trained homing pigeons above thick ground fog from a plane. Birds were first trained to home individually from two ground sites, each ~15 km from their loft. Subsequently, trained individuals were released from a plane ~400 m above the terrestrial release sites during sunny conditions. This was followed by two experimental release conditions above continuous ground fog. Above the fog, pigeons cannot use a map or olfactory location cues, but may potentially use magnetic cues. When all landmarks were hidden by ground fog, pigeons showed poor orientation, tortuous flight paths, and descended through the fog within four min, advancing only ~0.1 km (0.1% of the distance) towards home. They subsequently waited for solar cues to continue homing. By contrast, when distant landmarks (alpine mountain tops) were visible, pigeons maintained strong homeward orientation, flying for 20 min and covering 96% of the distance to home, similar to ground-level controls (17 min, 80%). These results demonstrate that when known olfactory cues or local maps are excluded, distant landmarks-but not magnetic cues-enable efficient short-distance homing in experienced pigeons.