I have two hens that crow, but they don't do it all year. They seem to enjoy their vocals in the mornings and mostly during spring when hormones are fresh and frisky.
One is a Speckled Sussex, now age six years, and the other is a three-year old Cream Legbar. Linda the Sussex will go at it for about an hour in the mornings and she sounds like a young cockerel practicing, but she ends each series of notes with a long drawn out yodel with a credible vocal tremolo.
The Cream Legbar sounds very nearly like a regular rooster, though a young one, still not practiced yet. She also does her crowing first thing in the morning, but she will only let loose with two or three and she's done for that session.
Both hens are "all girl", no secondary male sex characteristics, and both are regular layers. And they don't do it because there's no rooster in the flock. Strawberry, the Cream Legbar rooster fulfills his role admirably and doesn't appear to mind two of the girls horning in on his act.