In terms of biology, the reason for this head bobbing behavior is because of how owl eyes have adapted to being nocturnal predators. Owls have amazing night vision, which comes from having an incredible number of rod receptors in their eyes. Rod receptors are extremely sensitive to low light environments. These are peripheral vision receptors that enable a wide angle of alertness for even the slightest movements, which is great for spotting tiny mammals as they graze along the tall grass in open fields.

The downside is these rods are basically color-blind and require lots of space in the eye for the high numbers required to work in near complete darkness. In order to fit the most rod receptors possible in their eyes, owls lose the ability to move their eyes in the sockets. The extra space enables them to fit more rod receptors, but it removes their ability to manually focus their eyes.

This means owls need a specialized strategy to help them judge distance that replaces movement of the eye with movement of the entire head.

One of the fascinating things about this is humans have this capability too, but most people never learn how to develop it. Humans typically judge distance with our cone receptors by adjusting the focus of our eyes along an axis of visual depth.

You can feel this subtle adjustment in your eyes by focusing your gaze on something far away, then move the focal point to something much closer and feel what happens to your eyes. If you’re attentive, you will feel a slight micro-movement of your eyes as they adjust.

This type of cone-based depth adjustment is something that owls cannot do so they compensate by moving their heads to generate the motion parallax effect (rod based depth perception)