I keep seeing people mentioning that they want high numbers of birds, but to be honest I don't see why.
For key species, the variety isn't huge. Having two species of macaw in one DLC would be akin to including two subpops of tiger or two brown bears, which we got in the base game, but you know what I mean. If they gave us the bald eagle and the golden eagle, as another example, to me that's the same thing. Both would be nice, but we certainly don't need both to cover a wide variety of birds in a pack. Honestly, eight species isn't that bad for "key zoo animals".
Wading birds like ibises and cranes, and waterfowl, could be habitat animals, so if the pack is focussed on flying birds, it's likely these will be skipped in favour of more conventional species. I'm also more convinced than ever that birds will be the start of a new period in PZ, rather than the end of the old period. So there's no reason to think this will be our one shot at birds and then it's over.
Time will certainly tell, of course, but I'm confident.
You may be right in terms of absolutely essential bird species, 8 wouldn't be bad for start.
But I still would like to have way more. I think, unless many here I'm a bit more interested in birds, so I would love to have more diversity. Especially parrots and songbirds are such large groups with great variety and very colorful and appealing species, so I think we need more than e.g. one macaw species.
I think almost all bigger and even smaller zoos have tropical halls with smaller free-flying birds like crowned pigeons, fruit doves, small parrots, turacos, bali starlings, other songbirds like glossy starlings or fairy bluebirds, small duck species, ibises etc. etc., so I think a larger choice of species would be great to enhance realism even more. A tropical hall without any free flying birds doesn't feel complete for me.
So besides the classical, bigger birds like macaws, owls or birds of prey I would be great to have more smaller birds.
For birds of prey I agree with
@KönigDerKaffeebohnen and would choose the bearded vulture. It doesn't look like a classical vulture but would be a really great choice because
a) it has an interesting conservation story and is an example for a reintroduced species thanks to captive breeding
and b) this species naturally lives in Europe, Asia and Africa, so it has a very flexible use for everyone who builds zoos with geographical areas.
I think as a second bird of prey the bald eagle is likely, especially the American players want this species and it's also quite common in European zoos.
For owls the snowy owl is a no-brainer in my opinion. They are among the most common birds in zoos, in Europe alone there are almost 500 zoos with this species and I think the situation in the US is similar.