Yes, boys and girls, birds talk with their mouths full. Or at least Warbling Vireos warble with their beaks full.
I don't have video to prove it, but I watched a Warbling Vireo yesterday who continued to sing mellifluously as he whacked a large caterpillar against a branch until it cooperated and agreed to be swallowed. The bird stopped singing long enough to ingest the bug and wipe his mouth, before moving on to stanza two. Then he proceeded to catch another caterpillar just in time for the refrain. Impeccable timing.
The Warbling Vireo is a pretty nondescript bird--overall light gray and dark gray--but what it lacks in eye-catching plumage it makes up for in pipes. In my experience it's one of the singingest songbirds in our midwestern trees. And I can forgive the Warbling Vireo for its poor table manners, because its song is so beautiful--a long and rambling warble, the way a Robin's song meanders, but more gentle, and usually ending on an up note.
The Warbling Vireo can be hard to spot, because it often hangs out high in the canopy. But when I do manage to get a photo, the effect is usually soft and beautiful, because the light is filtered by the leaves. I really like this plain bird with its less than perfect etiquette.
And while it may sing with its mouth full, I have never seen a Warbling Vireo in a bad mood.