It's been a while since I've seen swans.
Years ago, my then-little dachshund Sharikov (named after Bulgakov's protagonist in the Heart of a Dog, because I adopted him) lept into Lake Ontario on a number of occasions to chase up to half a dozen (!) swans at once. He did so fearlessly--barking at them right out of the water--despite their obvious attempts to lure him deeper into this enormous lake and conceivably peck him to death and drown him.
Yet during my recent trip to Russia and Northern Europe, I've encountered white swans practically everywhere I went (insert Leda-and-the-Swan joke here). This particular specimen is from Moscow--the Patriarch's Ponds, specifically--where he torpedoed local ducks as aggressively as Torontonian swans did my Sharikov.
Of course, the Patriarch's Ponds are a magical place.
Bulgakov simply recorded this fact in the Master and Margarita (here I come full circle!). So, perhaps, this wasn't a swan at all, but Woland himself or, at least, one of his underlings. It would be quite fitting for the ugly and fanged Azazello to masquerade as a beautiful swan waiting for Margarita to give her diabolical lotion--so that she could turn into a witch.
But these are just my suspicions. ;)