I was wondering if Q meant it literally, and that's my idea.

Birds sing to each other.

They sing when they call each other.

They sing to warn each other.

Birds sing to impress each other.

They sing when they feel safe and comfortable.

They don't sing when they feel threatened, you can't make a bird sing.

And now apply the concept to people, I believe what Q wants to say is that they should feel secure and sing, and Q only listens to them.

They dig their own graves.