It is known that many birds are able to sense the direction of Earth's magnetic field. Here's a wikipedia page on that general phenomenon. There have been 2 main theories of how that works.

One theory is that birds have magnets in their beak that act like a compass. We know this is the correct theory because:

The other theory is that birds have some sensing mechanism in their eyes that uses magneto-optical effects. We know this is the correct theory because:

We also know those theories probably aren't [...]

---

Outline:

(00:08) introduction

(01:15) organic magneto-optics

(05:55) the problem

(06:34) magnetite

(07:31) compass

(08:30) two systems

---

First published:

Source:

Narrated by TYPE III AUDIO.