I have two male runner ducks who are 3 months old. I've noticed that when they're swimming in the pond and they might be excited by the other ducks they will do something odd - kind of push themselves up, then put their heads down towards their chests and make a really strange noise - I hesitate to call it a bellow, because it's not THAT loud, but it's definitely weird and nothing I've ever heard from my female ducks before. I had ordered females, but these guys didn't arrive as ordered, so I'm enjoying seeing the differences between them and my girls. They love to stand up tall and look big and it seems like they give me a "report" everytime I come out. It's kind of cute. I hope they turn out to be nice boys when it comes to mating season....