A carrier pigeon can take a letter home even if you take it to a strange place thousands of miles away. A few turns of coils are wound around the pigeon’s head and neck to supply power with a small battery, and a uniform additional magnetic field will be generated on the pigeon’s head. When the current flows clockwise, the pigeons released on cloudy days will fly in all directions. This shows that pigeons navigate by geomagnetism. So how do pigeons navigate by geomagnetism?
Some people regard the pigeon as a semiconductor with a resistance of 1000 Ω. When it flies in the earth’s magnetic field, its wings cut magnetic lines of force, resulting in induced electromotive force (i.e. induced voltage) between its wings. Pigeons fly in different directions. Because the direction of cutting magnetic lines of force is different, the direction can be distinguished by the magnitude of electromotive force. However, the test shows that the additional magnetic field does not affect its flight in sunny days, which shows that geomagnetism is not its only compass. Originally, the pigeon can measure polarized light. On sunny days, it can choose the flight direction according to the position of the sun, and the body clock can correct the movement of the sun accordingly. It must be noted that when the current flows counterclockwise, it can fly home whether it is sunny or cloudy.