Since smaller birds got brought up from the OP, I'll go with it, and Macaws, even though my most singularly wanted bird is not a Macaw. I would gladly take any number of Macaws Frontier would give us, I'm talking happily having a dozen or more. But I don't think we'll get any, which is a shame since they are the birds that I think of whenever "birds in zoos" are mentioned. These would be exhibit birds and while I had hope for them before, I don't anymore.
For what could be possible in game right now, it's wading birds. Ibises, Spoonbills, Herons, the Shoebill, Storks, Egrets and more Cranes. There's others I want but collectively that's definitely my most wanted group that could be done at this time, which we know it can be since we have the flamingo and red crowned crane.