On the news ticker on the official website, it now says that work has started on the removal of the old aviaries around the zoo. I assume it refers to the ones near Islands in Danger so we'll know soon enough if a lawn is going to replace them
As has previously been said, there is a lot of empty space within the zoo now (particularly in the central area of the zoo)...
I wouldn't hold your breath on it being replaced by anything other than a lawn at this moment in time!!