For those who think this is mink, I really don't agree........
Go to youtube, search "Mink Attack" by Dave Raven (there's more than one clip with that title). My understanding is from my friend who is an old old-timer trapper. Also, another swan breeder who is also an old timer (he actually put chickens twice in a cage with a mink and observed how the mink killed the chickens--yeach!) gave me a very detailed description of the whole thing!
Both of them say that the minks kill by biting the bird on the back of the head in a specific spot that seems to render the bird immediately immobile and non-resistant. The minks often don't even do anything else at all to the bird and just leave the carcass with only a bite mark to be seen.
I looked into this after the first of 2 juvie swans I sold ended up like this at the edge of the buyer's beautiful pond. A couple months later, the second youngster was found dead-- also perfectly intact with only a bite on the back of his head. I have told many folks about this experience which was even sadder because I had been so thrilled when these babies went to this new home. The buyer's pond and layout were so beautiful and seemed like paradise for them--then it turned out to be quite the opposite.
This is why I make such a big deal about predator protection when I post my swans for sale!!