We have 13, one male for sure. About half of them do the head bobbing thing to no one in particular, they will do it with me as well. They are about 16 weeks. It IS cute. Now when we let them out to their pool, it's a mad rush (just a kiddie pool) and they all mount on each other, it's like a duck orgy. No one cares whether they're boys, girls or whatever. One will stand on another and then another and so on and so on, like a train. This lasts for a couple of minutes and then they settle down and "behave." No one has gotten hurt, think they are just letting their hormones run free, and once they get through it, they don't do it again til the next day. Love that head bobbing though, sure it is a sign of affection and recognition. It looks like your two have a thing for each other, hard to tell from the video whether they are male/female or whatever, when they get excited they tend to "floof" a bit. But looks harmless and appears they are good friends. No harm done, right? As long as it's not aggression, looks normal. Have seen mine do the same, our male has picked a favorite female, and they kiss and nuzzle a lot, and make cooing sounds, and know for sure one is he and the other is she. Think you are safe!! Enjoy it and take pictures for your duck baby album.