There it is! I was looking at Philadelphia Zoo photos before my trip and I had a feeling they had this aviary. So when I couldn't find it, I figured I mixed it up with the National Zoo. This looks beautiful.
The aviary was closed on our visit but a very friendly volunteer had great things to say about it.It sure looked impressive and we really enjoyed the bird building.
It will need major upgrades to make it accessible, but you are right it is very disappointing to go into this amazing and beautiful structure to see a couple of ducks.