Audubon magazine explains why owls bob their heads.
All these varied head movements help the owl judge the position and distance of things around it—essentially, to triangulate on objects, including potential prey, and to build a composite picture of its surroundings. This head-bobbing helps make up for an anatomical limitation: An owl’s eyes are fixed in position, so they simply can’t move the way our eyes do. To look up, down, or to the side, an owl has to move its head. They have very flexible necks and can do 270 degrees of a full head turn, looking over one shoulder, around the back, and almost over the opposite shoulder. And after a few of these head-bobs to triangulate on their prey, they rarely miss.
Read full story at Why Do Owls Bob Their Heads? | Audubon