The most useful habitat birds we could get imo are these:
1. Rhea genus: greater rhea being the one with the widest distribution, but rheas in general being an important and iconic species from SA. It is also the only last big ratite left. It is very versatile in terms of habitat (grasslands, deserts, wetlands, alpine) and relations with other animals, as it can be easily used for mixed exhibits with things like capybara, tapirs, anteaters, patagonian mara, etc which makes them very common in zoos as well.
2. Grey crowned crane: not only is a very iconic african bird, that can also be put in almost any mixed african enclosure or aviaries, but it also has conservation value being endangered. Also common in zoos.
3. Any duck: i prefer mandarin ducks for being very ornamental and unique among ducks but also very widespread around the world while at the same time representing east asia. But really we just need at least one duck to fill in ponds and mixed wetlands or tropical exhibits. Mallard is probably the prototype duck, very common everywhere both in the wild and in captivity so might be the prefered choice for most. White face whistling duck has the advantage of being found both in Africa and SA which increases it's value and use. Muscovy duck would be great just because would work as 1. SA rep 2. Domestic duck.
4. Any pelican: objectively the great white is the best choice here because of it's wide distribution, but personally i rather get a brown pelican species, bumping up the caribbean a bit more while serving as LATAM rep once more. But a pelican is a must have, specially now after the swan rig. Is one of the few ABC animals left that the wider audience is very familair with for how iconic they are. And they are aslo common in zoos.
5. Galliforms: i think there are three basic options here: 1. a pheasant for Asia 2. a guineafowl for Africa 3. the turkey for NA. All of them very representative of their regions and commons in captivitiy. Pheasants of any kind would add a lot of colour to our Asian sections, with the golden pheasant being the most iconic of them probaly common both the zoos and farms, great for Chinese gardens walkthroughs or mixed asian forest enclosures/aviaries and such. The guineafowls for savanah exhibits mixed in with monkeys or ungulates would be fun. The turkey is one of the last icons of NA not ingame alongside with the ABB and the bald eagle. Being the latter not possible nor expected at the moment the turkey is a great option for a NA bird. I would add the cappercaillie to this list as once again an iconic bird for Europe (although the swan was really the main one) but might be a bit of a obscure choice for most non-european people. Still very wanted from my part.
6. A stork: Europeans want the white stork and i get it as it is such as pelicans and ducks, a bird archetype that is ingrained in popular culture around the world. Yet a marabou stork or saddle billed stork would do wonders for filling the blank space in the roster that are the ground dwelling african savanah birds. They are also common in zoos in mixed exhibtis and look rather unique. Regardless a stork is needed.
7. An ibis: scarlet ibis is probably the most versatile and demanded of the bunch and my prefered choice. Is small but colorfull, distributed across South and Central America, great for mixed aviaries and tropical houses and common in captivity. That said the northern bald ibis is a very unique looking bird that holds great conservation value and would something that is not an ungulate to North Africa.
8. Roseate spoonbill: another unique group of birds not represented in the game that would work great as habitat species in PZ and offers an alternative to the flamingo. It's range covers from southern US to Argentina so once again it would give us a panamerican species that would work great for both NA and SA based zoos. Also good for mixed species enclosures and aviaries. And i prefer it over the American flamingo.
9. A heron: any would do. Maybe is not a zoo animal per se most of the time being just a wild resident of zoo lakes and ponds, although some more flamboyant species sure are part of collections officially. Great white herons, or little egrets are probably some of the most common birds in wild and we could use them for open grasslands and wetlands enclosures. Grey herons are cool and common in Eurasia and Africa and bit more visually interesting so it has my vote to represent the group.
10 The burrowing owl: once again a panamerican species the ranges from Patagonia to the US. Like the name suggests would be our second burrowing bird, only this time representing a new group that we might not get in other way: the owls. Repeating myself for like the fourth time, owls are alongside pelicans, ducks, storks, etc. a very popular well known group of birds with tons of cultural relevance. Maybe the burrowing owls are not the most well known among the owls but they are funny, goofy, cute and extremely entertainign and they would fit nicely as habitat animals in PZ. And i'm sure they would be very loved by most playeres.