A husband in a medieval farming town decides to follow his wife on her nearly daily walk in the country, thinking perhaps she's seeing a man out there. He finds her by a small mountain stream. To his surprise, she is naked with her legs spread on a granite boulder, getting her pussy eaten by a lovely little green-haired dryad.
He almost moves to intercede afraid his wife has been enchanted or bewitched by the fae folk when he hears his wife's moans. "That's it you little forest slut, eat my pussy."
He, as well as his wife, sees the little dryad is furiously fingering herself. "I'm hesitant to do so, but I suppose a deal is a deal. Tomorrow I will bring my husband out to stretch out that little pussy, so you can fulfil your little prophecy or whatever." He watches, stroking himself through his britches as his wife gets closer and closer to an wonderful orgasm, the dryads fingers pistoning into her wet warmth.
He uses their distraction to creep back off into the wood, looking forward to the next day with all his heart. He's no idea what this prophecy could refer to, but all he can think about is getting into that tight little pussy.
Oh and I had a groovy little thought about that prophecy too. You could change it to a nyad and tie it into the birth of Lancelot and play with the Aurthur legends a bit.
The title makes me think of bird-watchers (or "birders" as I believe they call themselves). Peeping through binoculars into trees looking for more bird species to check off their lists. One dude is looking through a tree, and zooms in on a window on the other side of the tree, which has a beautiful young woman inside the house, walking around naked. He is mesmerized, and watches until she leaves/goes to bed/gets dressed or whatever. But he comes back the same time each day (or evening) and watches her, becoming more obsessed all the time. Eventually...... well, it could go lots of ways.
You could also include a lot of bird puns. Like feathering the nest, keeping an eagle eye out, early bird gets the worm, kill two birds with one stone, a little bird told me, a bird in the hand, what are you chicken?, etc.