Hello Francoise, There are few sources for information on singing
birds.  The most scholarly is Chapter XVII, vol II of the book Le Monde
Des Automates by Chapuis and Gellis.  A translation from the French into
English is printed in the Musical Box Society (US) in the Autumn 1996
issue of their journal.  Other information is scattered throughout the
literature.

If you have specific questions, I will try to answer them.  I am not an
authority on the subject, but I repair and restore bird cages and boxes
and can give you the benefit of my experience.

Good luck,

Moe Goldy