Mallard plumage

I have been thinking about plumage in Mallards, for several reasons.

I have been watching the older family (hatched in early May) of Mallard ducklings grow up and have been looking for any signs of the appearance of sexual differences in plumage, so far unsuccessfully.  Research turned up the concept of a "drake feather" but this develops only after three or four months of age. That age appears to be when differences in bill colour and breast feathers occurs, as well.  I could not categorize any of the ducklings as to their sex.

I have also been watching the adult male Mallards moving into eclipse plumage.  Their bright glossy green heads are fading to a dull purple.

Soon the only clue left that will differentiate a male Mallard in eclipse from a female will be the greenish cast of its bill.