I was planning to day the exact same thing. I know multiple zoos have had problems with people throwing coins into their Duck ponds, and having a bridge over the exhibit would only exacerbate this problem.And the issue of visitors dropping or even throwing things into the exhibit
Another zoo with a bridge over exhibits is the Marco Polo Trail at Roger Williams Park Zoo. The snow Leopard and asiatic black bear have same-level viewing, but there is only elevated viewing from the boardwalk for the Cranes, red panda, and Takin.