@Mr Gharial Actually this aviary is not that old at all. It was opened in 2003 along with the current hippopotamus house and the tapir and pelican (currently harbor seal) exhibits.
Originally at this site there were a stable and paddocks for water buffalo and Malayan tapir at the backside of the 1950s hippo house.
This aviary was actually originally built as a walk-through aviary with ground level viewing only, and originally housed a more general wetland bird collection. However in 2015 the marabous and vultures moved in, and I believe not too long after that (would have to check my photos) the upper level viewing area was added.
If you search for hippo house aviary in this gallery you can find some images of the original set-up.
