I have a feeling that the canal bridge may be listed, i'm not sure, perhaps this is linked with the tunnels? What about the Snowdon aviary, is that listed?
I think the old listed raven cage is (or at least has) been used as a bird feeder for the resident sparrow population. The mesh on the cage are big enough for the sparrows to hop through, and it stops all the food being stolen by bigger birds.
I'm glad the Cat Terrace is not listed, perhaps they should get it pulled down before it is! I've been looking at an aerial photo of the terrace. It's a bit of a maze, and I always feel a little disorientated in there. There is a lot of public space which is a bit of a waste, and I think there is some off show areas in there. There are three enclosures with primates in, and three with cats, but the actual floor space is even bigger than that. If the area with the cat terrace aviary was included, and the pelican and flamingo pool, there would be room to build a massive tiger space (upto 6x what the current exhibit is), as well as enclosures for the gibbons and langurs. I dare say there may even be room for a new tapir or anoa enclosure (although almost certainly not both).
Below I've drawn round the area roughly covered by the cat terraces and flamingo pool. The blue line is the area that the tigers have currently. One thing that is very promising is the amount of mature trees in the area.