Just adding a little more to what the others said: when we look directly at things our brain processes a slightly-more-than 2D image because of the position of our eyes. This is leaving out other factors that our senses send signals about to perceive depth.

So, think of it this way: you have a box in front of you. But, how do you know it's a box and not just a cardboard cutout made to look like one? You walk around it or adjust your neck. Same concept for birds.

Contrast this with chameleons (can move their eyes independently) or animals whose eyes are more exposed (and have near-360 degree vision, save for what's under their bodies). These animals don't need to move their heads as much.