How can I tell the gender of a young duckling???

[olp_63]

We were going to purchase a very young duckling (probably about a week old or so) from our country fair today. They are Mallard ducklings, but we would really like to try and get a drake (male).
Is there ANY way of telling (or even guessing) what sex a duckling is at such a young age?

Here is a pic of what the ducklings look like:

 

[herechickchick]

It has to do with the sound they make when you remove them from the others but I can't remember what sound. Someone will come along with the answer.

 

[perfectly_polish]

You really can't tell when they are that young. When they get older the female will have a very loud quack and the male will have more of a whisper. When they are around 4 months the males tail will start to curl.