After the final cloudfall, you will receive access to River's End which is on the North side of the map. (Marked #14)

Once you've traveled atop the nearby mountain, you should find a bowl with two Male Winnows inside, they will not be able to be charmed.

In order to fill the prerequisite, you will have to retrieve and drop off 20 Male Winnows via charming. If they are returned to the bowl 1 by 1 rather than an entire flock, they will remain there until the cutscene procs.
Male Winnows are found circling the river water and accompanying rocks between River's End and Shelter Grove - they will not run away.

Upon returning with the total of 20 Male Winnow, they will commence a ritual to summon a Female Winnow, locking you in a cutscene.

If the cutscene did not proc, you need to collect more Male Winnows.

Don't forget to charm her after discovering!