I have three five week olds, mostly feathered, and ONE is quacking, so I know I have at least one female. These are my first ever ducklings BTW. The female and another have dark bills, and the third has a lighter olive green bill. Of course they are all identical in feathering except for the bill color. The adult Rouen drakes seem to have lighter greenish bills, and the females dark bills. Is this a sex difference you can see at five weeks of age?