system
Given the following post, respond as if you're commenting on Reddit:
human
How do birds detect magnetic fields to know which way is north and south?
gpt
Their eyes can see it. Light induces the formation of radicals (unpaired electrons) in part of the bird's eye that are affected by the earth's magnetic field. Their orientation to the poles affects the signal, so they can very accurately tell north and south. Without it, their travel over water or at night would be fatally off course.