Sekai Meisaku Douwa: Hakuchou no Mizuumi

When Prince Siegfried spots a beautiful swan with a crown on its head, for some reason he feels compelled to follow it. He discovers that the swan is, in fact, a princess named Odette, who is under a wizard's curse that causes her to become a swan by day. The wizard, Rothbart, wants to keep Odette to himself, and the only thing that can break the spell on Odette is a man who truly loves her, with all his heart and soul...

(Source: Anime News Network)