A small songbird that has never migrated can leave its birthplace and reach a wintering area thousands of kilometres away. It does so using several independent navigation systems at once.
A magnetic compass gives direction
Birds detect the Earth's magnetic field and use it as a directional reference, which works at night, under cloud and over featureless ocean.
The field's inclination, the angle at which it meets the surface, varies with latitude, so it can indicate how far north or south a bird has travelled as well as which way it is heading.
Evidence points to light-sensitive molecules in the eye and to iron-containing structures in the beak, and the two may serve different functions rather than duplicating each other.
The sun and stars supply calibration
Day-migrating species use the sun's position combined with an internal clock, since the sun's bearing only indicates direction if the time of day is known.
Night migrants orient by star patterns, and specifically by the point around which the sky appears to rotate, which stays fixed as the night progresses.
Young birds learn that rotation point before their first migration, which is why the star compass depends on early experience while the magnetic compass does not.
Landmarks matter more with experience
First-year birds tend to fly a genetically inherited direction for an approximate duration, which brings them to the right region rather than the right place.
Experienced adults navigate to specific sites, returning to the same woodland or estuary in successive years, which requires a learned map rather than a compass alone.
Coastlines, mountain ranges and river valleys act as leading lines that concentrate migration into recognisable corridors, and these are where migration is most visible from the ground.
Weather determines when the systems are used
Birds wait for favourable conditions before departing, because a tailwind substantially reduces the energy cost of a long flight.
Adverse winds and heavy rain can displace migrants far off course, producing the concentrations of unexpected species that birdwatchers describe as falls.
Navigation systems allow correction afterwards, and displaced adults reorient towards their destination while young birds more often continue on their original heading.
Artificial light and structures interfere
Bright light at night disrupts orientation in nocturnal migrants, drawing birds towards illuminated buildings and holding them circling until they are exhausted.
Collisions with glass and structures are a major mortality source during migration, and reducing lighting during peak passage measurably lowers it.
The vulnerability follows from the biology: systems tuned to faint celestial and magnetic cues are easily overwhelmed by strong artificial signals.