I'm very curious about a behavior that my Swedish Drake is exhibiting. I call it "whistle and hum" and it occurs when the drake is in the pond with two or three of his ladies. He starts by rising up in the water while giving a short whistle, then gives a low hum while dropping back down into the water. I've never seen any of the other drakes do this, and it only happens when he is in the pond. Any ideas?