Let us see how this happens. The road side telephone pole seen from the window of a moving car appears to pass more quickly than the faraway landscape for us. Similarly, when bird's eye move, closer objects appear to move at a faster rate than that of distant objects. Thus, birds can identify the object that is near and far. Birds adapt different methods to enhance the distance measuring ability. For example, some birds like waterfowl, shorebirds (bob) move their head up and down often. Pigeons move their head back and forth while walking. Even perched New Guinea Kingfisher, moves up and down on their legs before diving after prey, to estimate the distance. You can also see how this works by moving your head with one eye closed and observe the relative motion of close and distant objects.

‘Hawk-eyed’ is the term that accurately describes most of the birds. In fact, there is evidence that hawks can distinguish their prey at 2 or 3 times the distance that a human eye can detect. One of the reasons, for the birds having such visual acuity is by having relatively large eyes. For example, a starling’s eye accounts for 15% of the head weight when compared to human, whose eye weight is less than 1% of the weight of the head. Moreover, evolution has played a major role by arranging the structure of bird's eyes in such a way that each eye functions very much like a telescope.