My experience is the same as @hennible All of our Muscovies raise the feathers on their head, but I'm not seeing as an agressive trait as much as a social trait. And here's where things got really confusing for us - when we introduced 3 new ducklings to an existing flock, all three gravitated to the one drake we had. I'm not sure how relevant this is, but our existing flock was still young enough that we still couldn't tell the difference between our ducks and drakes. But what ended up as our big boy, was the one all three ducklings stuck the closest to. The ducks wanted nothing to do with the youngsters. And as far as aggressive behavior, neither of our males are agressive to us, just to every other living animal within breeding distance!! They are definitely horny young men, turning into randy old men!! Even our geese are not safe!!