Actually with the list provide not a single aviary could be made, you could make single species enclosures, but an aviary? You could mix the parrot species depending on what it is with the toucan and the ibis depending on what ibis, but that is it, and it would not even make for a good implied aviary to be honest.
I think it is going to be crucial to use the same definition of the word "aviary". If you only go by the definition of a multi-species covered habitat, I can certainly agree with your statement. I don't think there's even remotely a possibility that you can have a realistic scenario where you have enough birds to do that right of the bat.
Even if we go as far as 20 birds, there's no way you can make a DLC that would allow you to make a realistic amount of different multi-species aviaries. You'd either need to focus on one very specific region, or you'd have to compromise and end up with like 4 species for your multi-species aviaries for each region. Neither will ever satisfy people who are really into building this kind of aviary.
However, I am not talking just about multi-species aviaries. I see it broader; and I count single species aviaries as just as important. Plenty of which also exist in zoos today. I really don't find it fair to judge a possible bird pack solely on the basis of being able to make the big multi-species aviaries you are referring to. So that's why I went with the list. Choose the animals wisely, and some can be combined into a multi-species aviary. The other ones are single species ones, which is absolutely fine in my eyes.
For all intents and purposes, do let me state that I too want to be able to make those. But I also try to be as realistic with my expectations as possible.
I understand your point about receiving birds in future packs and I hope that is the case, assuming the next pack is even a bird related pack (if not the situation gets even worse) but how many more packs can we expect realistically after this? three more packs? 5 more packs?
What the amount of packs is going to be nobody can know, so that's honestly hard to say. I can't make any statements about it, because I just don't know. All we do know is that getting 8 birds now doesn't mean that we'll never get more birds.
I understand logistics and not being able to create additional assets when the platform can not sustain it, but then maybe creating a zoo game was not a great idea, because at the end they will end up with a game very unbalance and the legacy of it will be totally ruined for many of us, others might not care much. But like I said maybe all is not lost, even if all this comes to pass, modders could eventually become the legacy of this game and their names will eclipse Frontier's own in what is eventually done to allow players to create realistic zoos.
There's just a limit in the amount of money any company can throw into DLCs. For each company it's different, and for Frontier it's clear that their current approach is the way to make it profitable for them. They're not a charity, they're a company that needs to pay their devs and also make a profit. And I think that's fair. I also need to make money to pay my bills. I know people like to think that Frontier tries to do the "least possible for the most amount of money", but just objectively looking at all the content we get for free, I don't think that's even remotely true. They are by far not the kind of money-grab company like there are others.
At the end of the day, as with any game of any genre, compromise is the name of the game. There's not a single game in the world that can capture what a zoo is like, there's not a single development team in the world that can cover it all and provide it all. Compromises are going to have to be made. You're never going to be able to please everybody, so you can either accept that the compromise is there and work with it; or you can dislike the game for it. Both are equally fair choices, it's up to you which one of it it will be.
However, I do think that it's not so surprising that the games focuses more on (big) mammals. They are in the end the most popular animals amongst the general public, so the majority of people out there want those first and foremost. Birds are important to zoo fans like you and me, but I fear that the game's first and biggest audience is just animal fans in general who want to make their own zoo. And don't necessarily care about whether those are realistic or not.
I also really want to say that I completely understand where you're coming from and I think from that standpoint your opinion is certainly valid. To make an ultra realistic zoo, no game developer is ever going to be good enough and in the end we're going to look at modders for that. But I personally also believe that a good comprise can be made, and I also believe that it's possible to have that compromise in the 8 animal pattern.