@KevinB I can't recall any particular reason, not excluding that there is one.
Looking at ZTL however, I'm starting to doubt wether it's the male or the female which is the problem. A few years ago both Pairi Daiza, Planckendael and a private holder each had a pair. The private one and Pairi Daiza switched birds, and bred succesfully afterwards. Problem is that for both pairs, the same sex died around the same time. And to once again breed they would need to combine the fertile bird in Planckendael with one of the other two. But the management refused. And now, on ZTL it lists that the remaining bird in Pairi Daiza is a female. So either I misremember and it's the females which are infertile, or ZTL is wrong.
