Most birds sing with their mouths open. As such, when like the Carolina wren opens its mouth to call, the canal leading to the middle ear automatically closes. When this happens the air pressure increases in the middle ear. This, in turn, diminishes sound vibrations. In addition, the movement of the jawbone eases the tension on the eardrum.
Fascinating. Thanks for the info.
Bob,
I am glad you like it. Birds are so fascinating!