I have a pair of swans that have migrated to my lake annually for several years. They came back a couple of weeks ago, but last week I noticed only one swan swimming around. Now that one seems to have disappeared. Any idea what might have naturally caused this? I'm hoping no person or animal caused their demise. There are geese, ducks and a pair of blue herons also on this lake, but I also have coyotes and fox. Thank you.