But... by that logic basically no animal is essential .-.
Like let's take the giant panda for example, an icon of conservation and zoos that everyone agrees is a must have for a zoos game. Do all zoos have them? Barely, so by your logic they are not essential... nor gorillas... nor elephants...
Also regarding birds, so what if they are extremely variable? how does that matter at all? The general public does not care in the slightest what specific species we get, be it a red macaw or a blue macaw, the point is getting a macaw rather than not getting any. We need them! Obviously you will never be able to add all the species people want so that is not something they should care about at all as long as the species they add are useful for most. Parrots, pelicans, ducks, birds of prey, toucans, cockatoos, they are all essential animals for zoos. ZT1 had them, ZT2 didn't have them because of technical limitations but fortunately thanks to mods we got them, ZT3 had them again, even Wildlife park 2 had them?? (crazy)