Yeah, I agree on how fascinating the regional differences could be.
I actualy searched about the variety of Swans and I'm amazed what I found; there's actually a Black Swan while there is another type that looks like a duck. I believe the differences happen because of the environment and the kind of diet that they consume.
Same goes to another animal species like Flamingo, the ones that are colored pink is because of the specific type of algae that they consume in wetlands. It's the kind of algae that has high concentration of beta-carotine. This fact could be related to how swans developed different kind of features compares to other swans found in other region. It's indeed fascinating :)