In (a): the bird is looking for an object.
In (b): the bird has grabbed an object
Psittacines have an excellent field of vision in front, above and behind their heads. However, they are unable to see what is going on below their beak, where all the manipulation is taking place.
The diagram shows us how it works. You can see that where the binocular visual field ends, (area shaded in blue on the sketch), the areas outside of it are located under and behind the head. In these, the birds are completely blind. It's a blind spot for them.
To overcome this sensory failure, they are therefore forced to tilt their heads so that the studied object is at an angle of 90 °. Nevertheless, once the object is caught in the beak or the paw, it is an exploration of a purely tactile order that begins. The beak, tongue and toes of the legs compensate for the ineffectiveness of the eyes.
This peculiarity distinguishes psittacidae from other birds according to the researchers, since only they exhibit this characteristic. On the front of the head, their binocular field of vision is wide and they have good near sight around the head.