A map made of magnetism
The homing pigeon (Columba livia domestica) possesses a remarkable navigation system that relies on sensing the Earth's magnetic field. This ability, known as magnetoreception, provides the bird with both a map and a compass. Researchers, including those at Cornell University, have investigated this sense for decades, showing a biological system. The pigeon's "map" appears to use the intensity of the geomagnetic field to help determine its geographic position.
Experiments show that pigeons are sensitive to very small changes in magnetic intensity, on the order of nanoteslas. This allows them to detect the subtle gradients in the Earth's magnetic field. Evidence suggests that magnetite-based receptors, located in the upper beak and innervated by the trigeminal nerve, are responsible for sensing this magnetic intensity information. When this nerve is anesthetized or severed, a pigeon's ability to discriminate between magnetic fields is impaired. This information is then sent to the trigeminal brainstem nuclei, which show increased neuronal activity when a pigeon is exposed to changing magnetic fields. Studies releasing pigeons in areas with magnetic anomalies—places where the Earth's field is distorted—found that the birds' initial orientation was often confused, suggesting they rely on a predictable magnetic field to navigate.
Alongside the magnetic map, pigeons use a magnetic compass. This compass is thought to be based in the bird's eye and relies on a quantum mechanical process involving a protein called cryptochrome. This light-dependent mechanism allows the pigeon to "see" the direction of the magnetic field lines, providing a directional heading.
The olfactory debate
While the magnetic sense is well-documented, it is not the only tool in the pigeon's navigational toolkit. A competing and complementary theory suggests pigeons use their sense of smell to create an "olfactory map." This hypothesis proposes that pigeons, at their home loft, learn to associate specific environmental odors with the wind directions that carry them. When released in an unfamiliar location, they can theoretically determine the direction home by identifying the local scents.
Two main models describe how this might work. Papi's "mosaic model" suggests pigeons build a map from a patchwork of familiar odors within a 70-100 kilometer radius of their loft. Wallraff's "gradient model" proposes that stable, long-range odor gradients can be used for navigation over much larger distances. Experiments have shown that impairing a pigeon's sense of smell can severely disorient them, lending strong support to the importance of olfaction. Some studies even suggest that for homing from unfamiliar sites, an intact olfactory nerve is more important than the trigeminal nerve associated with the magnetic map. Further research at Cornell by William T. Keeton suggested that pigeons might also use very low-frequency sound, or infrasound, to orient themselves. The current understanding is that pigeons likely use a combination of cues—magnetic, olfactory, and solar even acoustic—integrating the information to achieve their impressive homing feats.