Hi Bob,
 I noticed this when we were there the other morning; in fact Rob  Hilton and 
I discussed this, as we both noticed the lack of nail on the hind  toe. But, 
what I also noticed, was that when the duck perched in its normal  upright 
position on a rock, the hind toes (both) made contact with the top flat  surface 
of the rock (the hind toe was vertical, and just the tip made contact).  Rob 
and I discussed the possibility that perhaps there actually is a nub of a  nail 
but it stays worn down by its contact with rocks(or whatever) when it  
stands. We could not see if there was any hint of a nail through binoculars, but  
from the side view it appeared not to have any. If you have any photos of it  
from the back, with a view of the end of the nail, this might help.
 
Rick Sussman
Ashton,MD