Nothing special at all, really, besides the rare species housed.
That is not how I would describe Stuttgart. Yes there are plenty of standard aviaries that are in cases too small. Good news for the birds is that the renovations of the subtropenterassen have started, so the right half (the part without the kaka) has recently been emptied.
But the Freiflugvolieren as well as the Amazon house are some very good exhibits.
Stuttgart has about 4 times more bird species than Nuremberg and it is one of the biggest ones in Europe, with plenty of rarities. And while I always advocate that quality is just as important as quantity, Stuttgart can offer both. Without a doubt the single best bird exhibit in these 2 zoos is the bearded vulture aviary which also houses black stork & rock pigeon (currently no nutcracker or choughs though). It is a former brown bear enclosure:
@ralph
The Desert house is also a very nice exhibit, but houses only 3 bird species, though they are interesting passerines: trumpeter finch, red-cheeked cordonbleu & firefinch:
The 3rd highlight is the manatee house with free-ranging tanagers, doves, honeycreepers, euphonia and some ducks:
Apart from that the bulbul in the carnivore house are a nice touch as is the mix of red-billed hornbill & dwarf mongoose. The big walkthrough with waldrapp etc. is also nice. But that is mostly it. I personally think that many of the owl and bird of prey aviaries are quite ugly and small / too small, as is the hyacint macaw aviary (though with good breeding results). So the Wilhelma is not the only one with that issue.
My bird highlight in the Wilhelma are the Freiflugvolieren which is a mix of 4 walk-through aviaries with multiple side aviaries attached. I really like the flowing design of concrete and steel, which is softened by the good planting. This complex alone has some 70 species, which is more than the whole of Nuremberg and a good many other zoos. Here are some pictures:
The Amazon house is also very nice for birds with some 10 species free-ranging including a large breeding group of yellow oriole
Designwise the aviary for steamer ducks & caracara is also a highlight:
Then there are also plenty of aviaries that are on the small side of acceptable and in some cases just clearly too small (roadrunner, many of the larger parrots). These problems even arise in the newer exhibits like the birds & small mammal house:
This is for a pair of roadrunner (an additional pair is kept behind the scenes, where they have a large off-show breeding centre)
This is how the subtropical terraces look (or used to look for a part of it). It has some 30 aviaries, which are all quite low and might be fine for the smaller parrots, but not for the kea, kaka or the other large species kept here
@Maguari
The Wilhelma has loads of other aviaries throughout the zoo. Most are just fine, but a particularly noteworthy one houses Congo peafowl, together with whistling ducks & robin chats:
@Cichlid
@Philipine eagle
Nuremberg and Stuttgart are 2 of my favourite zoos, but unfortunately for Nuremberg birds just aren't their strong suit, while they are for Stuttgart. So I have no problem voting 4-1.