Most large-scale mutations aren't advantageous. In fact, they're often lethal, since it's so much easier to break something than it is to fix it. In this case, to be this far off in plumage from anything we know well, you'd have to have several mutations at once, or one with some rather odd pleiotropic effects. The first choice is unlikely to say the least, and the second one seems likely to cause some other unsavory physiological effects.
Hybrids, on the other hand, are a well-documented occurrence in quite a few bird families, sometimes to the point where we birder's have to throw our hands up in despair. (Olympic Gull, anyone? Kumlien's?)