birds' vision

This though occurred to me today when I was watching a Suphur-crested  
Cockatoo on the local oval. It was approaching a potential piece of food  
and it turned its head to look at it closely, then turned its head the  
other way to look with the other eye. I realised that this is a common  
movement with other parrots and birds that I see regularly.
My question is, why do they do it? Do birds with non-binocular eyesight  
have differential vision in each eye (different focus length or something?)  
If they don't, why do they look with each eye in turn, as what the see  
would be the same?
John Leonard
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www.birding-aus.org
birding-aus.blogspot.com

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