I think this is because one of the giraffes is pregnant and they want separate areas when the calf is born. It looks really awful though. The Nyalas can walk right through the fence. I don't think the Ostriches would be able to. There is an Ostrich on the other side of the fence in the photo, by the keeper; and a Nyala in the shelter, which had walked through the fence just a moment before.
