After some discussion it looks like my first visit will be reduced to a day trip and I'll only be in for the afternoon.
I am taking notes into account. Herpetarium and River's Edge best or later in the day. Red Rocks and birds earlier in the day. I'm actually quite excited about the aviaries even though I'm often poor at identifying birds, they're quite unlike what we have in Chicago.
Thank you for asking. Besides various hoofstock (kudu, lechwe, goral, babirusa, urial, banteng, gazelles) at Red Rocks, mostly the sun bear, jaguar, spectacled langur, and lemurs especially sifaka. Black mamba is the main reptile and cormorant the main bird outside the penguin exhibit. Maybe king vulture?
Do they no longer hold tomistoma/gharial? I notice it's not on their website.
Cool! I will say that I am in the progress of compiling information about the rarities at stl but I can give you some more information about the others you've mentioned. I will preface by saying that I wouldn't get my hopes up too high for sun bear or spectacled langur. With the former, I have rarely seen them active, and they tend to hide, so its hard to get a good viewing experience. With the latter, just because of the nature of the exhibit complex they're in, Canopy Trails, they aren't consistently viewable, unfortunately. I honestly usually skip the Primate House but whenever I do go in they don't have an indoor viewing area for the species, meaning that if the spectacled langurs don't want to be outside, they won't be viewable. Its still worth checking the Primate House though, as I said I haven't been in there in a while and things do change.
As for the lesser kudu, ironically enough given the zoo's logo, there are no males on display as of my last visit. There is a large yard though near the Cypress Swamp where the females are mixed with saddlebill storks and varying species of gazelles (as per last visit, there was a singular Speke's gazelle in there).
The Soemmering's and Speke's gazelle vary a bit in location. They have been placed in various mixed exhibits but are also sometimes solo. As of my last visit, there are two small yards behind the giraffe yard. Aesthetic-wise, these yards are probably among the weaker exhibits at the zoo, unfortunately.
The addra gazelles are a bit more consistent location wise. They've been in the mix in the giraffe yard before, but I haven't seen them there in about a year. There is also a yard near the Red Rocks restaurant and tables right by the viewing area. The menu selection isn't as big as it is at Lakeside or Cafe Kudu, but if you could find something there that you'd be alright with you could watch them while you eat.
Weather permitting, all the gazelle species at the zoo are pretty consistently viewable. If you go on a colder day, they tend not to be out though. I think they're supposed to keep them inside when it below 50 or 55 degrees out, but I am not entirely sure on that one. Provided its a warm day though, I usually do see the gazelles.
There are two babirusa yards, one closer to where the addra gazelle and one near the giraffes and bantengs. I'd recommend spending more time at the latter, since those individuals seem a bit more active. The two individuals in that yard are very interactive with each other. Do be aware though that giraffe feeding lines often spill out into the viewing areas for that yard though, as well as the Speke's gazelle yards, so I would avoid it at giraffe feeding times.
The Nile lechwe is another one of the species kept behind the giraffe yard. He's pretty consistent. He likes to musk himself in his droppings so he rarely has clean fur. He's a pretty majestic animal if you can look past that though.
Jaguar is another inconsistent one. When he is out and moving or even just lying in an open spot, he is quite popular, but he does have big viewing areas so its not so bad. He's another one I would try to come back to multiple times throughout the day, because he usually is out at some point, its more of a matter of when. Jaguars and lions have a big enough viewing area so that even during high traffic times at the zoo, you can have a pleasant viewing experience, despite the popularity of the Big Cat Country complex. Its really the smaller species in the mesh-topped exhibits (Amur leopard, snow leopard, puma) that can be difficult and frustrating to view during high volume times at the zoo.
Coquerel's sifaka is usually viewable. They're pretty active in Canopy Trails and usually have an indoor exhibit in the Primate House too.
Black mamba is one of the more active reptiles, which isn't saying too much but still. It doesn't garner much more than average attention from guests when its resting, but when its moving about people can crowd around the exhibit so it can be hard to view. If you visit the Herpetarium during lower traffic times, this isn't so much of an issue though.
I absolutely love the STL zoo's cormorant! He's probably one of my favorite individual animals at the zoo. His name is Jack, and he's a rescue. He was caught in a fishing line and had a wing injury, so one of his wings had to be amputated. He just has such a big personality. He lives in the Cypress Swamp. He is sometimes hiding in the tall grasses so you might have to stick around for a bit for him but its worth the wait. Cypress Swamp is such a great exhibit to just hang out and get immersed in.
As for the penguin exhibit, as popular as it on this site, I honestly don't spend too much time there. I go to the zoo primarily for photography, and that exhibit doesn't really lend itself too well to photography (at least not with the equipment I have). I do spend quite a bit of time outside with the Humboldt penguins though. Unsurprisingly, this part of the zoo gets crowded and there are sometimes lines to get inside so its another thing I'd try to do at lower traffic times. Also, I recommend bringing a face mask. Its a great exhibit but the smell is not the best part, and the face mask can mute that part a bit (though not completely).
King vulture is outside during warmer months but in the Bird House for winter. Right now, its mixed with red-billed blue magpie. Its actually neat because when its outside, its right next to the cinereous vulture, so you can actually see a representative of both the old and new world vultures side-by-side.
As per my last visit, tomistoma/ Malayan gharial is still very much at the zoo. Its the first exhibit to the right when you enter the lower level of the Herpetarium through the outside. They don't have an Indian gharial (or gharial proper) though and haven't in recent history.
Also, I'll give more detailed explanation of traffic patterns at the zoo, as I realized I probably didn't communicate that as clearly as I could. On weekdays, the first hour or two is pretty slow. Then the traffic gradually increases up until about noon. After that, it stays relatively stagnant until about 2:00 or so, when people, especially families with young kids, start to clear out.
On weekends, it is much busier overall but the mornings stay slower longer. Its really after noon when its starts to pick up, and it doesn't clear out until pretty late.
I will also mention two things since you said you're interested in birds. The first thing is that the zoo does have a 15 minute free-flight bird show. It consistently features and American barn owl, black-crowned night heron, African gray parrot, Abdim's stork, and bald eagle. It also sometimes features a tawny eagle (another rarity) and red-tailed hawk. The second is that Forest Park, in which the zoo is located, is an excellent place to see native birds, so I recommend staying in the park after you leave the zoo. To that point, watch the lakes at the zoo for native birds. There are so many great egrets, American wood ducks, and black-crowned night herons that the zoo actually signed them there. I've also seen snowy egret, green heron, hooded merganser, gadwall, and a subadult yellow-crowned night heron, to name a few.
