-> I'm sure they can, they just won't tell US how they know.
GRIN!  There are SEVERAL "Wives Tales" around here about it but I have
no faith in any of them.....
My vet, is SURE he can't tell.  Our only avian vet is about a hundred
miles away.  I don't think I'd survive a hundred miles of cooing. 
-> If he's searching through his food so determinedly, he may be looking
-> for a little variety in his diet. What do you feed him? It may be
-> time to expand his eating horizons a bit.
About the only thing he actually EATS is maize, I recently gave him
several whole maize heads and he studiously picks the individual grains
off and throws most of them away.  I've also recently gotten him some
chicken scratch which has all sorts of whole and cracked seeds in it and
he eats the naize and throws most of the rest of it out of the cage.
Last week I locked all the doors and let him out of the cage, and in no
time at all he was on the floor eating what he'd thrown away with great
gusto! 
-> like the bird-eating vacuum cleaner...
Yes.  I have a Great Dane eating vacuum cleaner!  My old "Big Dog" used
to love having his back vacuumed. (But then he liked EVERYTHING!)
 ^..^
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