I assume that this thing is battery-powered, old, and German.  This
would mean that it has a small, old, battery-powered motor inside to
run the little pumps.  If you're lucky, the motor's pivots require only
lubrication.

In any small DC motor the shaft should turn freely, and can generally
be checked for this by shaking the shaft such that it tries to move in
and out of the motor--that is, the shaft should rattle quite easily.
Be careful if you're planning to lubricate it, for the motor brushes
won't work if oil gets into them.

If the motor can't be repaired easily there ought to be others that
will fit inside and do the job.  If the motor is okay there may be
rubber drive parts that have deteriorated in some manner and must be
replaced or rebuilt.

My qualifications here are somewhat scant because I've never worked on
a singing-bird device.  However, I did restore a Greisbaum whistling
figure (spring powered but equipped with German wiring for the lamp-post
lamp) and I've worked with DC-powered audio devices forever.

Mark Kinsler