http://agathe.gr/democracy/athenian_currency.html
Athenian Currency Many of the specialized administrative boards have left material traces of their activities. Most prolific of these were the moneyers, or Overseers of the Mint. Throughout her history ... Because of the representation on the reverse the coins were called “owls,” and Aristophanes thus refers to coins made from silver mined at Laurion in his advice to the Athenians in the Birds: Little Laureotic owlets shall be always flocking in, You shall find them all about you, as the dainty brood increases, Building nests within your purses, hatching little silver pieces.