This is the 13913th most frequent German word.
flattern
"Flattern" means "to flutter" or "to flap."
Here, 'flattern' is used to describe the gentle motion caused by the wind.
Die Blätter flattern sanft im Wind.
The leaves flutter gently in the wind.
In this case, 'flattern' indicates the rapid movement of wings typical of a bird's flight.
Der Vogel flattert aufgeregt hinaus.
The bird flutters out excitedly.
Here, 'flattern' is metaphorically used to describe the feeling of nervous tension.