system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a pigeon's head move when it walks?
gpt
Notice that the pigeon has its eyes on the side of its head, not two at the front like we do. They are effectively looking out each side, but with one eye. Two eyes are required for depth perception. You, with binocular vision (two eyes at the front), can tell how far away something is because your brain calculates the angle yours eyes make when you look at an object.

Pigeons can't do this, because their eyes can't see the same objects. They're on the opposite side of thier head. To percieve depth, the pigeon bobs his/her head. This makes close objects move more than distant objects in the pigeons view.

Try closing one eye and moving your head side to side. See if you can tell how far away things are, relatively. Now try bobbing your head forward and back. It doesnt work, right? You need to move in a sideways direction relative to the line of sight for that eye.