When do ducks get their 'waterproof' gland? And do they use it by instinct or are they taught by their mother?

i havent been able to find exactly when they start to use the gland, but have been able to find out that their mother waterproofs their down until they molt, so i imagine they learn the behavior from her.
from what ive been able to find, it would make sense that the gland would start working around the time they molt.

i have sent an email to a local waterfowl sanctuary, hopefully someone there will answer.

By 6 weeks of age ducks are pretty well feathered out, their oil glands are working and they're ready to swim.

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