Frogs

One infallible sign that spring has come is the appearance of frogs in my pond. This year they have come a week earlier than usual, following a very mild winter.

This year there are about thirty of them, all getting very excited and making a lot of noise. Very soon the pond will be full of frogspawn

Does anyone know why American frogs are supposed to say "Ribbet?" This is quite a difficult sound to make, involving complex movements of the lips and tongue. My frogs never say anything except "Kaaa! Kaaa!"