[Animals] Migratory birds fly thousands of miles without ever getting lost, and scientists think part of the answer is literally in their eyes

[ScarletWrench]

Some migratory birds travel astonishing distances each year, and remarkably, many species complete their very first migration entirely alone, with no adult bird to follow or guide them, relying purely on instinct and internal navigation systems rather than learned knowledge passed down socially.

Researchers believe birds combine several different navigational tools simultaneously. They can read the position of the sun during the day and the pattern of stars at night, essentially using a built in celestial compass, and many species also appear able to sense the Earth's magnetic field directly, a capability scientists link to a light sensitive protein called cryptochrome located in birds' eyes. Some researchers believe this magnetic sense may actually let birds perceive magnetic field lines visually, essentially seeing directional information overlaid onto their normal vision rather than sensing it as a separate, distinct feeling.

Birds also appear to build and rely on mental maps using landmarks, coastlines and even smell in some species, layering multiple redundant systems on top of each other. That redundancy likely matters enormously in practice, since cloud cover can obscure the stars, overcast days can hide the sun, and any single navigational tool failing under real world conditions would be catastrophic for a bird crossing an ocean if it had no backup system to fall back on.