Cincinnati is not the only zoo with such pinioned birds mixed with hoofstock. Besides how certain are you they didn’t want them to breed? I can’t find it in the AZA mixed ungulate manual.Because Cincinnati went out of their way to prevent any breeding in that specific habitat?
Of course you need to be able to compare them with proper exhibits for these birds to draw my conclusion, pretty hard when you’re living in the US where pretty much everything still gets pinioned and just thrown out in mixes with large mammals. I’ve seen both outdated mixes with hoofstock and modern aviaries here in Europe, and it’s quite clear which ones work better. There are always the few mixed exhibits here and there with ungulates that do work well. But when species like marabou which are mostly kept in mixed savannah’s have most of their breeding happening in a handful of aviaries, you don’t need to be a genius to know which type of exhibit works better. For many of these species, good breeding results can also be obtained in open-topped separated exhibits, but often at the cost of some breeding/rearing success, physical health or lost behaviors.
There are always exceptions to the rule. Perhaps it’s just a bolder pair of vultures, the oddball couple so to say. It likely also helps that sitatunga are not the biggest nor most aggressive antelopes. On the other hand, in Pairi Daiza they have this nice large group of marabou storks in with their sitatunga and they have failed to breed for nearly 15 years whilst the newer, younger and smaller unpinioned group in Antwerp had success in less than 5.Curiously the Zooparc de Beauval has kept for many years a pair of Griffon Vultures with a breeding herd of Sitatungas, Emus and Marabou Storks (what a strange mix !) in an enclosure.
And the Vultures bred nearly every year !
Also, once a pair starts breeding the chances of them breeding again are highly elevated. To properly look at the success of a mix, you’d need to replicate it multiple times with different individuals. This doesn’t only apply to birds, but to any mixed exhibit. The fact that it works somewhere is no guarantee that it will work most of the times nor that it will work forever.