Hello George.  The advice given here by Mr. Morgan, and also by Craig
Smith, is quite correct for the older singing-bird boxes, but not for
the newer boxes, which use neoprene discs instead of wooden bellows.
I believe that the neoprene discs are available from Black Forest
Imports, and possibly others.

Regards,

Bob Loesch, in beautiful Lake County, California, USA
http://www.cuckoobob.com/