Is there anyone familiar with the tweeters found in the base of many
birds-in-trees automatons, sometimes called "bocages"?  They consist
of a small bellows with an attached thin brass plate with a small hole
in it that makes a simple tweet sound when pumped.  When paired with
an alternating second bellows, they make a tweet-tweet sound.

I would like to make a replacement for a missing one but, after many
unsuccessful tries, I need advice about the theory behind them.  The
diameter of the hole, taper of the countersink, thickness of the plate,
and any other pertinent information would be greatly appreciated.

Moe Goldy