“Dolphin on back of duck; one head serves for both.”
So instead of two heads are better than one they decided to go with one head is better than two? ¯\_(ツ)_/¯
“Orifice in tail of duck and round opening in head which allow passage of air through body and up hollow neck. Small lump of clay confined in head of dolphin forms a rattle.”
So the duck bit is a whistle, and the dolphin bit is a rattle. Neat.