Hi - After having had a laugh at the other replies I thought it best to warn you that ducklings change their feathers, and whilst they are doing it they are difficult to pluck owing to the young "pin" feathers and stubs.
Duckling grow their frame and their first set of feathers in about 8 weeks or a bit less which is when most commercial ducklings killed - this explains why even a large looking shop duck will only feed 2 or 3.
They then have a growth pause whilst they grow their adult feathers. Once this is done they begin to put on flesh If you are intending to skin them then just go with the weight plus how much meat is on the breast.
Ducks are bad enough to pluck without making it harder than you need to

All the best
Sue