Barnacle Goose Brenta leucopsis, Bernache nonnette.
Barnacle Geese nest only in three places in the world, the Spitzberg, Novaya Zemlya and eastern Greenland and are very conservative in their wintering sites. Therefore it is easy to understand why people used to marvel at their arrivals and departures to an unknown (at the time) destination.
Two explanations were favoured, both resting on metamorphosis, either that it was generated from a shell, the Bernacle or Barnacle Lepas anatifera or that certain trees, resembling willows, more particularly in the Orkneys, produced a special kind of fruit, containing the embryo of a goose which, when ripe, fell off into the sea and took wing (HARTING provides us with an illustration from Gerard's Herball, or Generall Historie of Plantes, 1597).
This adds another dimension to Caliban's fear which is now much more understandable as we see that if a first metamorphosis has already occurred for the barnacles, a scientific truth of the time, why not a second:
And all be turn'd to barnacles or to apes
(TMP 4 1 248)
Shakespeare makes the most of every image, all the details taking part in the creation of the atmosphere he wants to create. A comical touch is also added, as in the eyes of the audience, Caliban is already more or less an ape.