I know I am super late to this thread, but hey, rather late than never.
I recently found this going through old media of Pretoria Zoo.
View attachment 657292
It is from 1989, but it was a special release of the zoo's magazine for their 90th birthday showcasing the progress over those 90 years. (It is not long at all and barely says anything but still, they tried)
Translated from Afrikaans to English:
Yellow-billed Oxpecker
After it became apparent that the status of Yellow-billed Oxpecker became uncertain in some parts of the country (South Africa), the National Zoological Gardens (of South Africa) started with a breeding program that led to the first ever chicks being hatched, under our care, in captivity.
I am unsure when in those 90 years it happened, since it doesn't specify, but I am sure it must have happened towards the end. I would love to find more information on this, but I am yet to find anything of use.