I once encountered such a bird call on a theater pipe organ.  Before the
air reached the pipe, it traveled through a tube containing some sort of
oil.  The air made the oil bubble a bit before the air reached the pipe
or pipes.  The result was a bird call.

Bruce Clark

 [ One I saw used glycerin for a very "juicy" chirp.  The assembly
 [ was shiny metal and resembled a miniature teapot (or oil can).
 [ -- Robbie