Okay, I get where this is confusing. Yes, flying is a vertical motion that is definitely akin to swimming as far as moving in 3d space.
The difference is that Planet Zoo defines enclosures as walls (that can be damaged or climbed) and a floor/ground that is impenetrable. Birds would require 3d bounding in a way that seals and penguins don't have. There's basically no "ceilings" to habitats, so unless the collision boundary for walls is infinitely tall (which it isn't if animals can climb over them), then that's going to be a whole thing.
And that's just basically putting a lid on the box box.
Actual flying and perching animations might not be a huge thing. But let's consider perching. You add a tree branch to the iguana exhibit, there are spawn points where that iguana can appear. For birds to fly point to point in a habitat, means they need to have defined perch points. The trees that exist in-game right now don't have that. They'd either have to add/define perches to all the trees that exist, or birds would only be able to land in trees added in the pack that introduce them.
On top of that, there'd need to be multiple sizes of perch points. A space were a small bird could perch wouldn't be suitable for something like an eagle. So there'd need to be "layers" to the trees to differentiate where birds could land based on size.
And I didn't get to things like netting, domes, special doors, etc. My local zoo has a double door system that allows guests to enter the aviary section while simultaneously keeping birds from getting out. But there's not really a mechanism in game for that. Guests in-game aren't supposed to be in an enclosure and that could be a whole thing. Special doors, special staff, new behaviors, and almost certainly a lot of new objects.
And also, there's an element of scale. The game can only zoom in so far, and most small animals are exhibit animals. Up until they added prairie dogs, I was legit expecting the game to skip small animals. A lot of the nuance of having birds is going to be lost from the overview perspective. I can see large birds like hawks, eagles, vultures, owls, parrots, flying foxes being easier to implement than smaller birds.
It's not that I think they need to be a whole new game, just that there's a lot of little things to check off for birds to be in-game and behave correctly, and there's probably a little more to go into flying behaviors than there is for swimming. The game already defined water. There is a boundary, and the animal is basically either in water or out of water. A bird or bat, even perched, is always in airspace.
And I hope I'm wrong. I hope it is as easy to implement birds as it was to add seals. I just don't feel like it will be.
