Is it a new addition to the zoo? I certainly didn't notice it in there a year ago, and I'm sure I would have noticed the sign if not the bird. I did visit in winter though - was it kept in the outside aviaries and therefore be off-display in the colder months?
This species is kept there since at least 2011.
I haven't seen it either yesterday, so I am assuming it knows where to hide from the public or left the zoo since last year.
It was and still is not displayed on the signs.