Birds move their head in different directions besides their eyes. Birds move their neck very often to see any threat or danger or any hunter is attacking it. Also, some birds can't move their eyes enough to look around so they move their head to see around.
Binocular Vision:
Bird eyes don't move in their heads the way human eyes do. Thus, while those birds lack binocular vision, they have 340-degree peripheral vision, aiding them in spotting predators on the approach. So with its fixed eyes on the sides of its head, a robin must tilt its head in order to look down to the ground.
Birds have bigger eyes as compared to the other animals for better visual of their prey or predators from longer distances as they have bigger eyes they can't move their eyeballs so to have a look on both the side birds move their neck very often and other reason for this is they have more number of bones in their neck. So the neck is very flexible and can do fast movements.
Human eyes vs Bird eyes:
Bird eyes differ dramatically from human eyes. First, birds' retinas have about three times as many sensory cells as our human retinas have. Thus, like a camera with three times more pixels, birds enjoy much keener vision than do humans. Owls, specifically, have large retinas that give them not color, but maximum black-and-white vision in very low light. Eagles, if they could read, would be able to do so from a newspaper a football-field length away.
The satisfying head stabilization:
Several treadmill experiments suggest that most birds like pigeons stabilize themselves by bobbing their head back and forth. This explains why birds like pigeons and chickens have a super cool stabilizing skill. They stabilize their neck by holding their vision locked to an object and thereby balance themselves as they move. This is known as the optokinetic response. There are two phases observed in such a movement.
When the bird looks at an object while they move, they fixate their vision on that particular object for a while to get a clear view of it. This is called as the hold phase. When the rest of the body catches up and the bird moves, the bird then makes a sudden saccade eye moment–by turning the neck–to fix on a different object to compare the information and perceive depth. This is called as the thrust phase.
While the reasons for these rapid movements are still unclear, in a nutshell, the long necks of birds aid them in moving their eyes swiftly to balance themselves and perceive depth.
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