Do you mean to people or other birds? A drake duck may try to breed chickens hens, if he doesn't have a mate. Since their "parts" are different, he can kill or severely damage a chicken.
Instead of buying ducks, I went to the nearby feed store to buy feeders and lamps and I saw little cute chicks so I bought 7. The store owner said I got a barred rock and a black australorp, but I can't tell the difference. Can you help?