Q:

Why does the bird explode when Princess Fiona sings a high note?

I've never quite understood the scene in Shrek where Princess Fiona explodes a bird with her voice. 
Do birds inhale while singing? Did her high note hit the resonant frequency of the bird? Is there anything to this other than incongruity?

A:

Whereas a high frequency would usually shatter glass, often implemented to emphasize dissonance, the glass has here been replaced by a bird, something that can create high-pitched and dissonant sounds of its own, taking the trope a step further.
It's also typical for the ogres - as referred to regularly throughout the films - to have a detrimental effect on things we find classically beautiful or romantic, here directly shattering that iconic image of (DreamWorks' rival Disney's) Snow White or Sleeping Beauty.

Now imagine the bird exploding

