Ducklings at that age cannot be sexed by any visual or voice characteristics. They can be vent sexed, but it takes practice and experience to be able to do it, do do it correctly, and above all not to injure them in the process, which is fairly easy to do, especially for the inexperienced. I suggest that you pry open your wallet and invest in a good book. If you have this question, surely you'll have others, and the two best books out there on keeping domestic ducks are Storey's Guide to Raising Ducks by Dave Holderread and The Domestic Duck by Chris and Mike Ashton. They're not that expensive, and both have detailed illustrated instructions on how to vent sex.

And, if you can't afford the books (try Amazon.com for used ones) then your local library may be able to get them on inter-library loan from somewhere alse in the system. I strongly suggest you get at least one of them, if not both, as you will probably come up with other questions, and it is really nice to have to go no further than your bookshelf to know whether this (whatever 'this' is) is something you CAN do something about, something you SHOULD do something about, or if it is something you should now slightly panic about. Or if it is normal and to be ignored.... While I am not familiar with both books, I know from other areas that having 2 sources is a lot better than having only one.

Maybe ask the feed store to help you by keeping track of names of others that would either go half/half with you on a straight run, or is also purchasing a run - that way when their sex can be determined (drake feathers) you can swap out. The feed stores we have worked with have been happy to help in any way, particulary about linking people up.

Another option would be to fore-go the feed store altogether and order directly from a breeder rather than from a hatchery (i.e. Dave Holderread's - in Corvalis Oregon, by the way). You can write for a catalog and select what you like. They sex them for you and mail them(though you may be restricted to pairs with some breeds). You'll likely get higher quality birds AND have a better chance you'll actually get what you ordered. Our geese weren't quite the breed we thought they were - no fault of the feed store who ordered them for us.

Just a suggestion.

Thank you everyone. We've decided to just order a straight run.

Found the Storey Guide to Raising Ducks at the local library - very good book.