I have 2 female ducks about 7 months old. Recently when I try to pet them they have been vibrating their head and neck and then hitting my hand with their head as hard as they can. They will do it over and over if I let them. What the heck are they doing? This is really odd especially for one of them, she would normally stay away from my hands. I'm not sure if they're happy to see me or telling me to go away?