The obsession started once we went to the Nature and Science Museum. They have dioramas of animals and Leland loves to run around them and look at all the animals. For some reason the bison became his favorite. Now when I ask if he wants to go to the museum he keeps saying "bii, bii" over and over again until we get there and see them. Then he moves on to signing seal and making their noise. While we walk around and see his other favorite animals (the bears, the seals, the moose) he randomly interjects a "bii" so we always make sure to see them again on our way out. He also loves watching the video about bison and hearing their sound.
We also have the Rocky Mountain Arsenal Wildlife Refuge and they have real bison! We haven't seen them yet though. Next weekend we have a wildlife tour scheduled so hopefully we will see some. I can only imagine his excitement of seeing living bison.
And every time we go to the zoo he asks to see the bison, and I have to keep repeating that there are not any at the zoo. So Denver Zoo if you need a new animal to consider, consider the bison.