We went through this same thing with our first batch of ducks that we bought unsexed. We ended up with 3 males and 3 females, down to 1 male and 3 females now. If you buy ducks online you can buy them sexed and they come through the mail just after hatching - there's a special system the USPS uses so they call you as soon as they come in. It costs more to have them sexed but for us it's worth it because of how violent the drakes can get.