#bird sight

Everything comes back to the fact that birds have itty bitty heads and big giant eyes, if you know that the other details will follow. The biggest implication is that since the eyeball takes up so much space, there is little room left over for muscles of eye movement. This holds true as most birds cannot actually move their eyes to change their view point. However, lack of ocular muscles is not the only difference between birds and mammals. In terms of eye anatomy, bird eyes are not globes, they are more like flattened ovals. The flattened shape allows for a wider in-focus field of vision. I pulled this diagram off a British Garden Birds website to show a pigeon’s field versus a human’s:

In addition to this wide field, the eyeballs are held in place within the eye sockets by the bony sclerotic plate, so even if there were muscles, they still couldn’t move their eyes. 

In humans, our eye movements are controlled by the vestibular system which is the part of the brain concerned with balance. Balance has a fair bit to do with eye movement and image stabilisation. Here’s something a bird cannot do, focus on something in front of you, then move your head side to side. You can keep looking at whatever it is because your eyes move in counterpoint to your head. Since a bird cannot move its eyes, its head does what our eyes do and moves in counterpoint to its body to maintain its focus point. Head bobbing is kind of like an optical illusion because birds actually put their heads forward, hold them in place and move their bodies to catch up, their head doesn’t move at all. 

Different studies have tested different things concerning this kind of image stabilisation, head bobbing stops when the bird is flying, this is because it is not moving in relation to its surroundings and therefore it doesn’t need a stable image. One of the cooler things to come out is a theory on why wagtails and pipits wag their tails when they are foraging on the ground. It is thought that the tail wagging may cause minute viewpoint differences that enhance the birds’ ability to find food on the ground because they are seeing different images of the same area in quick succession. 

A while back (like two months back -.-) @zweetterion​ asked why pigeons bob their heads when they walk, I hope this answers your question and that it was worth the wait.

Sources:

http://blogs.telegraph.co.uk/news/peterwedderburn/100000937/why-do-hens-and-pigeons-walk-with-bobbing-heads/