Waldrapp ibis are down to 200-249 individuals in the wild and critically endangered. The zoo breeds them successfully in Europe on the Edge, and it's something of a risk moving them to the old condor aviary.
But this is a species bred by the hundreds in European zoos, so though endangered in the wild, they are commonplace in zoos, so especially a place like Chester could replace them with a species that is more in need of help...