This area is due to become part of the planned Savanna exhibit. I wonder if the badger exclusion policy is a response to the outbreak of bovine TB which caused the cull of the lechwe herd.
I got the impression that they didn't leave of their own accord in the first place. I believe there's a lot of badgers in the Clennon valley area. I imagine quite a few live in the area beside the zoo that is about to be developed as an industrial estate (work is being held up at the moment because of slow worms) so they'll be looking for new homes.
I see that the zoo still has a vacancy for a proofreader.