How do migrating birds find their way?

Birds use several sources of information at once. Researchers distinguish orientation, which works like a compass and tells a bird which way to head, from navigation, which works like a map and tells it where it is and where it is going. The compass cues include the sun, the stars and the Earth's magnetic field. Map-like information comes from landmarks, smell and possibly subtle magnetic signals. One Stanford summary lists five primary sources: topography (including wind shaped by landforms), stars, sun, magnetic field and odors, and says species may rank them differently. [1] [2]

Daytime migrants use a sun compass. It combines the sun's position with the bird's internal clock, so the bird can work out direction much like a living sundial. Captive birds whose light exposure was shifted to mimic another season flew in the opposite direction relative to the sun, which shows the link to the circadian rhythm. Patterns of polarized light at sunrise and sunset also appear to help, particularly for nocturnal migrants starting their flights. [1] [3]

Most birds migrate at night and use the stars. Until the 1960s scientists thought birds had an innate star map. In 1967 Cornell scientist Stephen Emlen showed that they instead notice the rotation of the stars, which gives a north-south axis. According to Te Ara, young birds must see the rotating sky before migration for this cue to become imprinted. [1] [4]

Birds can sense the Earth's magnetic field, which helps when the sun and stars are hidden by cloud. Altering the magnetic field around pigeons disrupted their homing. Two mechanisms are proposed: a magnetic mineral in the bill and light-sensitive components in the retina that may let birds effectively see the field. By sensing field strength and the angle of the field, birds may be able to judge position as well as direction. The Smithsonian source says both may help over areas with few landmarks, such as the ocean. [1] [3] [5]

Daytime migrants often follow mountain ranges, rivers, coastlines and lakes, and they can learn further landmarks over time. Raptors in the eastern United States follow the Appalachians or the coast, and some waterfowl follow the Mississippi. Smell matters too, especially over water. Seabirds called tubenoses use it to find food and return to colonies, and Scopoli's shearwaters with blocked nasal passages became disoriented over water but not over land. Te Ara adds that sound, including infrasound, may also contribute. [1] [3] [4]

Because migration spans long distances and varied weather, birds appear to calibrate their compasses against one another. Researcher Liedvogel is quoted as saying the cues vary in importance during a journey. Magnetoreception, for instance, is less useful during thunderstorms or high solar activity, which can disturb the magnetic field. Timekeeping for the sun compass is also complicated when birds change longitude and latitude. The sources do not fully settle which cue dominates in each species, and some sources describe several mechanisms, such as the bill mineral, as possibilities rather than established facts. [3] [4] [1]