The reason is that their best visual acuity is a small yellow spot called the fovea that contains the most sensitive cells of an eye. Most birds turn their heads because their eyes are not mobile in the eye socket. Bird eyes are larger than the opening that you see and in fact are as large as their brains. People have one fovea per eye. The foveae are sensitive because they contain the highest density of cone shaped cells.
Cones provide color vision in birds and people.
Rods are very sensitive to light but not color and are important for night vision. Owls see very well at night. Their eyes are enormous and full of rods and very few cones. They are nocturnal predators and can see in light at least three times dimmer than people can. Owls evolved to prey on small mammals, which were originally nocturnal to stay out of the way of the cold-blooded reptiles.