Every autumn, billions of birds leave their breeding grounds in North America and Eurasia and head for wintering areas hundreds or thousands of miles away. Many of them are small songbirds weighing less than an ounce, many fly alone, and most travel at night. Some young birds make the trip for the first time without any adult to follow. How they manage it is one of the oldest questions in biology, and the answer turns out to be not one mechanism but a layered toolkit of senses that back each other up.
The Scale of the Journey
Bird migration is easy to underestimate because so much of it happens in the dark. The BirdCast project, run by the Cornell Lab of Ornithology with partners including Purdue University and the University of Massachusetts Amherst, uses the same weather radar network that produces daily forecasts to detect migrating birds. On a typical peak night in fall, around 400 million birds may be aloft over the United States at once. In late September 2025, BirdCast recorded more than 1.2 billion birds flying south in a single night, the largest total it has measured since live migration mapping began in 2018. Long-distance champions such as the Arctic tern travel so far each year that, over a lifetime, their mileage could add up to a trip to the moon and back.
Moving that many animals accurately across oceans, deserts and mountain ranges requires two separate abilities. Researchers distinguish between a compass, which tells a bird which direction it is facing, and a map, which tells it where it is relative to its goal. A compass alone is enough to fly “southwest for a long time.” A map is needed to correct course after a storm blows a bird off track.
The Sun Compass
Birds that migrate by day can use the sun’s position. Because the sun moves across the sky by about 15 degrees per hour, a fixed angle to the sun only points the same way if the bird also knows the time. Birds solve this with their internal circadian clock, compensating for the sun’s movement as the day passes. Experiments in which birds’ internal clocks were shifted by keeping them under artificial light schedules showed that their chosen direction shifted predictably, which is strong evidence that the clock and the sun work together. Birds can also detect patterns of polarized light in the sky, which reveal the sun’s position even when it is hidden behind clouds or has just set, and many species appear to calibrate their other compasses at sunset using this polarized-light information.
The Star Compass
Most songbirds, including warblers, thrushes, sparrows and buntings, migrate at night. In classic planetarium experiments in the 1960s and 1970s, the ornithologist Stephen Emlen showed that young indigo buntings learn to orient by the stars. They do not memorize individual constellations so much as learn the point around which the night sky rotates, which in the Northern Hemisphere sits close to Polaris, the North Star. When Emlen projected a sky that rotated around a different star, the birds learned that star as “north” instead. This shows that the star compass is learned during a sensitive period before the first migration rather than fixed at birth.
The Magnetic Sense
The most intriguing tool is magnetoreception, the ability to sense Earth’s magnetic field. In the early 1970s, Wolfgang and Roswitha Wiltschko showed that European robins kept in cages oriented their migratory restlessness according to the magnetic field and that changing the field around them changed their heading. Their work revealed that birds use an “inclination compass”: rather than reading magnetic north and south as a hiking compass does, they sense the angle at which field lines dip toward the ground, which tells them whether they are heading toward the pole or the equator.
The leading explanation for how this works involves proteins called cryptochromes in the retina. When light strikes these molecules, it can create pairs of molecules with unpaired electrons, known as radical pairs, whose chemical behavior is sensitive to weak magnetic fields. This would explain a puzzling finding: the magnetic compass of many birds only works in the presence of short-wavelength (blue to green) light. In 2021, a team including Oxford chemist Peter Hore and German biologist Henrik Mouritsen reported in Nature that cryptochrome 4 from European robins is magnetically sensitive in the laboratory, and later evolutionary work from Miriam Liedvogel’s group at Germany’s Institute of Avian Research linked cryptochrome 4 to migratory behavior. Other studies point to iron-containing receptors linked to the upper beak and its nerves that may contribute to a map sense, though that hypothesis remains debated.
Smell, Landmarks and Experience
Seabirds add another sense. In experiments on Scopoli’s shearwaters, birds whose sense of smell was temporarily blocked could still navigate over land but became disoriented over open water, suggesting they use odor patterns in the atmosphere as part of their map. Over land, experienced birds also rely on familiar landmarks such as coastlines, river valleys and mountain ranges.
Experience matters in a deeper way too. In a famous study published in 1958, the Dutch researcher A. C. Perdeck captured thousands of starlings migrating through the Netherlands and released them in Switzerland. Adult starlings corrected course toward their usual wintering grounds in Britain and northern France. First-year birds, by contrast, kept flying in their original inherited direction and ended up in Spain and southern France. The lesson is that young birds start with an inborn program of direction and distance, while adults add a learned map that lets them navigate to a goal.
Genes Set the Plan
Breeding experiments with blackcaps and other warblers have shown that the urge to migrate, its timing, distance and direction are largely inherited. Crossing birds from populations that migrate in different directions produces offspring that head somewhere in between. This genetic program is what launches a young bird on its first trip; the compasses and maps described above keep it on course.
Why Navigation Can Fail
Because nocturnal migrants rely on dark skies and stars, artificial light is a serious hazard. Bright buildings and skyglow can draw birds off course, and they may circle lit structures until exhausted or collide with glass. This is why Lights Out programs ask building managers and homeowners to turn off unnecessary outdoor and upper-floor lights during peak migration, and why BirdCast publishes migration forecasts that help cities time those efforts. Disruptions to birds’ internal clocks from artificial light can also interfere with the time-compensated sun compass.
Climate change adds a different problem. If a bird’s inherited timing no longer matches when insects emerge or when wetlands thaw along its route, it can arrive at the wrong moment even if it navigates perfectly. Understanding how navigation works therefore helps conservationists protect stopover sites, reduce light pollution and anticipate how migration may shift in coming decades.
The Bottom Line
No single sense explains bird migration. Birds use the sun and polarized light by day, the rotation of the stars at night, a light-dependent magnetic compass that may rely on quantum chemistry, smell over the ocean, and memory of landscapes, all layered on top of an inherited plan. Each system covers for the others’ weaknesses, which is why a bird can cross a cloudy sky or an unfamiliar continent and still arrive close to where it started the year before.
Sources
- Cornell Lab of Ornithology: Record-breaking Night of Bird Migration Detected with Radar
- Live Science: How do migrating birds know where they’re going?
- Institute of Avian Research: The Cryptochrome Compass
