Every autumn, a songbird weighing less than an ounce can leave a forest in Canada and arrive weeks later in South America, often returning to the very same patch the following spring. How animals accomplish such journeys has occupied biologists for more than a century. The emerging answer is that migrants do not rely on a single sense but carry a toolkit of overlapping compasses — and switch between them as conditions change.

Researchers draw a useful distinction between a "compass," which tells an animal which way is north, and a "map," which tells it where it is relative to its goal. Solving both — what scientists call "true navigation" — is the deeper puzzle. Most of what is firmly established concerns the compass.

The sun and the stars

By day, many birds steer by a sun compass. Because the sun moves across the sky, this requires an internal clock: birds compensate for the sun's apparent motion using a roughly 24-hour biological rhythm, extracting a constant direction from a moving reference point.

At night, the stars take over. The classic evidence came from the American ornithologist Stephen Emlen, who in the 1960s placed indigo buntings inside funnel-shaped cages — now called Emlen funnels — with ink-pad floors that recorded the direction in which the restless birds tried to depart. Tested under a planetarium sky, the buntings oriented correctly, keying not on any single star but on the pattern of stars near the celestial pole — the point around which the night sky appears to rotate, a reliable marker of true north.

Strikingly, this is learned. Emlen found that young buntings acquire the star pattern by watching the sky rotate during their first summer. When he raised birds under an artificial sky rotating around the star Betelgeuse instead of Polaris, they treated Betelgeuse as north — proof the compass is calibrated by celestial rotation rather than hard-wired.

A compass in the magnetic field

Birds also sense Earth's magnetic field. In a landmark 1972 study, Wolfgang and Roswitha Wiltschko showed that European robins possess a magnetic compass — but one that works unlike a hiker's. Rather than detecting magnetic polarity, the birds read the inclination of the field lines, the angle at which they dip toward the ground. This "inclination compass" distinguishes poleward from equatorward, which is enough for migration. Recent work has suggested the field may also help supply a map, with birds extracting positional information from magnetic gradients — though how precisely they do so remains contested.

The quantum compass hypothesis

How does a bird detect a field far too weak to move a needle? The leading idea is the radical-pair mechanism, a hypothesis rooted in quantum physics. Blue light striking a protein called cryptochrome in the bird's eye is thought to trigger an electron transfer that creates a pair of "radicals" whose quantum spin states are sensitive to the magnetic field. The field subtly alters the reaction's products — a signal the bird may, in effect, "see."

The supporting evidence is suggestive but incomplete, as Scientific American and others caution. Cryptochromes do form light-triggered radicals, and a brain region linked to vision activates when birds use their magnetic compass. But the full chain from molecule to behavior has not been pinned down, and the precise mechanism of magnetoreception remains one of the most active open questions in sensory biology.

Smells, landmarks and putting it together

Other cues matter too. Homing pigeons appear to build an olfactory map from atmospheric odors, and near home they fall back on familiar visual landmarks — coastlines, mountains and rivers. Crucially, birds combine and recalibrate these systems, regularly resetting their celestial compasses against the magnetic field and vice versa. The result is a layered, redundant navigation system: when clouds hide the stars, the magnetic sense can carry on, and when one cue grows unreliable, another takes the lead.